Old Bridge
Located across the street from the Vatican City wall if you're following it from San Pietro to the Vatican Museum. Very cheap
The Vatican Museums have a reasonable cafeteria style restaurant, a bar, and a pizzeria, all of which are open during museum opening hours, and until about one hour after closing. See also Rome.
Seasoned travelers will know how to tip, and how to ask for the price at the counter, instead of buying at the over-rated 'sit-down' full-price. Better to tip and be welcome again the next day, than ridiculing everyone by paying triple or at least double.
Prati
Siciliainbocca
nice Sicilian restaurant in the quiet and somewhat boring part of northern Prati. Try the swordfish and the lemon sorbet!
Insalata Ricca
Part of a Rome-based chain, offers good salads and other food to both tourists and locals.