Mixed colour cherry tomatoes, one tub of either mini bocconcini or regular sized bocconcini cut into bite sized pieces, good EVOO and balsamic vinegar and a shake or two of Herb and Garlic Mrs. Dash! Win every time!

My go-to recipe for summer potlucks and barbeques is a black bean salad from Allrecipes.
http://allrecipes.com/Recipe/Black-Bean-Salad/Detail.aspx
It does take some time to chop everything up, but it's consistently been a hit. Plus it can be vegan-friendly, depending on the dressing used.

Note: I usually omit the jalapenos, but add a little tabasco sauce. I've also used bits of sweet pickle instead of pimentos.

my favorite cheap dish is the same idea - marinating fresh veggies for a few hours, then grilling them once you get to the party. always delicious, and at least in my circle, it's a dish so simple that nobody ever does it, and then everyone is excited to eat it! try asparagus with orange-flavored olive oil and parmesan, or sliced zucchini with mint, chili oil and pine nuts :)

I think vegetable salads that can sit out at room temperature for a while without being harmed (i.e. no lettuce salads that would wilt!) are great for this type of event. I made this cous cous salad recently that was fairly easy, and you could use whatever veggies are on sale at the market in it. If you are able to find the cous cous in the bulk bins it is often cheaper:

http://cupcakemuffin.blogspot.com/2011/05/israeli-couscous-salad-with-red-grapes.html

For later in the summer, this peach and tomato salad is SO easy and delicious - for maximum deliciousness and minimum cost, though, obviously wait until both are at peak season in your area:

http://cupcakemuffin.blogspot.com/2010/10/peach-tomato-salad.html

I second (or third?) the fresh veggies idea. A great addition to these is homemade hummus, which can be whipped up using a can of chickpeas AKA garbonzo beans, lemon juice, garlic, olive oil, and tahini in a food processor. To budget out the tahini, you can replace it with peanut butter or even yogurt.

We're always looking for good sides to go with BBQ.

This weekend we had one meal with grilled veggies and another with grilled fries. I don't go to the lengths that were previously mentioned with exotic oils (no offense). For us simple folk, some zucchini, eggplant and portobello mushrooms just marinated in some salad dressing for even as short as 30 min to an hour work great. The key is just not cutting them too thin so as to have them totally disintegrate on the grill. I did something like 1/3" width matchsticks for the zucchini and 1/3" rounds of eggplant. Three zucchini and a large eggplant were just enough for 6 adults.

The other meal we had this weekend involved grilled fries. Not sure where I saw the recipe for that, but basically just cut some Russets into about 8-way wedges lengthwise, oil and s&p and throw them on a hot grill. I added the additional step of blanching them first to take out some of the raw potato flavor and slightly cook them before grilling so as not to dry them out too much on the high heat of the grill, but either way should be fine.

I think with either side I spent a total of $15 and $6 respectively, considering I already had oil and a bottle of salad dressing on hand.

SMORES. Hands down. SMORES are like barbecue crack and insanely cheap.

Buy some cheap skewers, graham crackers, marshmallows, and hershey bars. Then be prepared for everyone to love you.

Simple grilled zucchini - small ones - cut in half and brushed with olive oil and grilled till they are very soft.

Melissa Clark has a really great marinated broccoli salad that uses sesame oil. It would be delicious at a BBQ because you make it ahead and can serve it chilled. It is in her newest book, In the Kitchen with a Good Appetite. I have made it several times, it's quick to put together and really tasty!

I love doing a combination of raw and blanched vegetables, served with Green Goddess dressing (where I use a lot of yogurt and not so much mayo). Mix and match textures and colors, and fun to showcase seasonal veggies. I also second the idea of potato salad as a popular, yet frugal choice. I make mine with homemade mayo, yogurt, mustard, green onions, parsley, and LOTS of fresh basil.

Along the same lines as the Melissa Clark recipe:
Barefoot Contessa's Sugar Snap Peas with Sesame from her first book.

Easiest ever, 3 ingredients, minimal prep, and gets better the longer it sits around.

Fruit salad or a fruit tray is always a hit. White bean dip made with a can of white beans, some lemon, olive oil, garlic and a handfull of spinach all whizzed up in a food processor is easy. Toast up some pitas, and you have a super easy and cheap appetizer.

A pasta salad with veggies and a vinaigrette dressing can be done fairly inexpensively as well.

The simple sassy slaw from Susie Middleton's Fast, Fresh & Green. Savoy cabbage stays crispy.

i just made a cherry, pecan, and feta salad - you can always replace the cherries with some other fruit that is in season to cut down the costs:

http://theactorsdiet.wordpress.com/2011/06/04/life-is-just-a-box-of-cherries/

I've been eating a lot of what Paula calls her South Georgia Caviar. Can be done with canned ingredients, but we've been using fresh (because it just tastes better.)

http://www.foodnetwork.com/recipes/paula-deen/south-georgia-caviar-recipe/index.html

We nix the pimentos and add okra, and italian dressing is fine but if we use it, we never use a whole bottle

I love oil and vinegar based pasta salads because you can use whatever you have in your house like peppers, onions, and tomatoes then top with a little oil, vinegar, and mustard whisked together!

I also love mixing up potato salads and doing a sweet potato salad instead!

These two are my staples and have been big hits at BBQs, both cool and refreshing -- and best of all, cheap and super-easy to prepare.

Arugula and Goat Cheese Pasta Salad http://cookieandkate.com/2011/arugula-and-goat-cheese-pasta-salad/#ixzz1Ke1t5JaD

and

Honey Apple Slaw
4 tbsp. Non-fat Greek Yogurt
1 tbsp. O Extra Virgin Olive Oil
2 tbsp. O Honey Apple Vinegar
Salt and Pepper to taste
5 c. finely sliced green cabbage (about ½ a head)
1 c. Julienned Fuji apples
½ c. roasted almond or pecan slivers (optional)

In a small bowl whisk together the first 3 ingredients. Add salt and pepper to taste.

In a medium bowl combine the finely sliced cabbage & julienned green apples. Just before serving, mix the dressing with the slaw. Sprinkle with the roasted almond or pecan slivers if desired.

Both recipes last several days (and taste better chilled).

I'm also a fan of grain salads -- easy, filling, and inexpensive. This one with the addition of goat cheese or feta is great: http://www.eatingwell.com/recipes/warm_quinoa_salad_with_edamame_tarragon.html

Corn and bean salsa is my go to - basically a can of corn, can of beans, chopped up red bell pepper, maybe finely chopped jalepeno, with italian dressing (of all things), mixed and let to sit over night. It's great with tortilla chips, and can be put on top of things (mmm, hot dog with the salsa on top!). Plus, canned things make it CHEAP!

grain or bean salads. using dried lentils or dried chickpeas makes it cheap. we do a couscous and chickpea salad for potluck bbqs, and people always ask for the recipe.
couscous (or bulghur), chickpeas, scallions or red onion. can put in red peppers (or leave out due to cost). dress with lemon, olive oil, curry powder, salt. you can improvise, but that's the basic idea.
lentil salads are also super easy and cheap - can even just do lentils, red onion, and a dressing. both are hits with vegetarians too.

Definitely give these homemade brioche buns a try! They are cheap and easy! They'll dress up any summer cookout.

http://thefauxmartha.com/2011/05/17/brioche-hamburger-buns/

Pasta salads that are balsamic & olive oil based are great and cheap! Chop up pretty much any leftover veggies in the fridge, add seasoning and away you go! But stay away from the mayo-based pasta salads. Much less healthy and in my opinion less yummy too!

My favorite room-temp salads:

- shredded/julienned carrots marinated overnight in lots of lemon juice and pressed garlic with a little salt
- chunked red potatoes - skin on - with lightly steamed green beans and quartered roma tomatoes in a mustard vinaigrette. Add some onions if you want.
- sectioned oranges (canned mandarins work in a pinch) and thinly sliced red onions in a lemon vinaigrette (basically lemon juice and olive oil whisked with s&p), which is great on top of field greens
- I also second the snap peas with sesame oil. I lightly steam mine (for like, seriously, less than a minute) then toss them in sesame oil, soy sauce, and a little grade B maple syrup. Sesame oil is pricier than olive oil up front, but is really useful when cooking asian-inspired food.
- peach and basil salad. Cut up half yellow and half white perfectly ripe peaches and toss with fresh torn or chiffonaded basil. Add fresh mozzarella pearlini if you want, but I like it better without.

I find that bbq potlucks are often heavy on the meat, bread, potatoes, and dessert, and light on fresh fruits & veggies. I like the lighter vinaigrette dressings for summer because mayonnaise is sometimes nice (creamy & cold), but can sometimes be way to heavy in the heat.

Carrots, potatoes, and onions are cheap - so they make nice bases! You could also bring potatoes, onions, a bottle of olive oil, and a roll of aluminum foil and do "hobo" packets on the bbq - just thinly slice up the veggies, toss with oil, S&P, and fold into aluminum packets and cook on the grill until done.

Good luck!

Mine is a pasta salad with two kinds of olives, green onion, rotini, cheddar and pepper jack cheese, salami, pepperoni, and a bottle of italian dressing. No mayo to worry about and this can be made in advance. Here's the link:

http://juju73.wordpress.com/2010/06/16/cookbookmania-potluck-pasta-salad/