Price aside, I was curious to see if the taste would differ and it actually did. I found myself missing the shallots. Here's where I found the main difference: shallots have a really nice way of incorporating themselves more fully into sauces or custards (such as a quiche) whereas onions, even if chopped finely, will largely maintain their shape so you'll have little bits of onion in your recipe. That's not what I was going for with the quiche recipe. I wanted the savory onion flavor without the chunks of onion, and that's what shallots did for me.
Speaking of flavor, I found that to be a bit different, too. The flavor of an onion, even a mellow white onion, is a bit more agressive than a shallot and it was much more present in the quiche. Again, not quite what I was shooting for.
A few years ago, Emma said of shallots: "Shallots have actually become our top choice for most preparations calling for raw onions. They are sweeter and more mild than either yellow or red onions, and have a pleasant crispness in salad dressings and grain salads." So while it's always nice to save a buck or two, I'm not convinced that shallots and onions are swappable in a recipe. If it's something forgiving like a chunky tomato sauce, sure. But if it's a dish with some sense of subtlety like a quiche, creamy sauce, or vinaigrette, adding shallots to the shopping list is the safest bet.
(Image: Faith Durand)