It can vary by season. Throughout the year, potatoes tend to be quite inexpensive, though a few nitpickers argue they are a starch and not a vegetable, and they say the same thing about corn. Right now, corn on the cob is about the cheapest vegetable per serving. Things like peppers, onions and tomatoes generally don't make for side dishes but rather are components in main dishes and sides. Right now, those are cheap, but when they're out of season and have to be brought in from elsewhere, they can get expensive. Right now, zucchini and summer squash might be the cheapest of all. Those absolutely humongously overgrown zucchini that are loaded with seeds and are pretty much best battered and fried or used for zucchini bread can be had 3 for a buck at the farm market. They must weight five pounds apiece.