Anywhere in Mexico, food, shoes and music is plentiful! It's funny to me that one of the first questions people ask is about food. It is never a problem in Mexico. Food is everywhere!
Fruit and vegetables can be found in the neighborhood tiendas; the main market (mercado); Vía Orgánica; and yes, at the big chain grocery stores. There are three big chain grocery stores in San Miguel de Allende; Mega, Soriana and the Bodega Aurora. There is a smaller one in Centro, Bonanza, which has a little of everything except produce and it has a limited meat selection.
You will never run out of restaurants to try, either. Many great chefs from Mexico City come here and open beautiful boutique restaurants. Any regional cuisine you have a “yen” for, you will find. The weather here is so gorgeous, you may tend towards all the outdoor dining that is available. Prices are from cheap to expensive. But for sure, you'll tend to want to try everything.
So how do prices compare with the US? Well, fruits and vegetables are phenomenally less; we are talking paying pennies versus dollars. They are weighed in kilograms and you'll pay just a few pesos. In the local markets, you can stock up your refrigerator for around $10. A jar of name brand spaghetti sauce is about $1.30 and a package of pasta $1.45. A can of soup, loaf of multi-grain bread and half-dozen eggs, less then a dollar each. A block of cheese would be considered an expensive item; but $2.70 to $5.40 for a block of cheddar still sounds cheaper to me then what you would pay in the US.
Household products are much cheaper, too. Shampoo, soap, and toilet paper for example; are about two thirds less. A four pack of toilet paper and shampoo is about $1.25 US. A pack of cigarettes is still under $2. A six-pack of tall beers is about $5.40. What you won't find too much savings on is hard alcohol.