Navona Notte, near Piazza Navona, is one of the best places in the centre for good quality and price.

For a sandwich, Aristocampo in Campo de' Fiori isn't bad. And, right on the piazza you'll find the Forno Campo de' Fiori, for terrific and affordable white pizza by the slice.

If you are really on a budget, but want a drink with dinner follow the street behind the church of Santa Maria della Pace until you come to a small piazza with a bar in it (its the only thing there).  For the price of just one drink you can help yourself to the buffet that is served between 7.30 and 9.30 and includes pasta, rice, dips, bread, vegetables, etc., then you won't need dinner.

Another cheap place to eat is in Parioli, and it's called Il Simeto (it is in Via Simeto!). It could seem a bad restaurant (old fashioned, small, the owner could even be rude), but food there is more than delicious: very large portions of pasta, good meat courses, and a wonderful pizza, all at a very affordable price.

A very good cheap place is close to Termini Station is called Sfizio Pizza in Via Giovanni Giolitti, 77 (corner of Via Gioberti). Menu items are  between 5 to 15 Euros. Very good pizza, lasagna, and chicken, and they have also English breakfast for the morning.

Great German and Italian food with excellent beer can be found at the Italian brewery, Birrera Peroni, near Trevi Fountain.

Termini Station itself is a good and cheap place to eat. A self service buffet, usually stocked with fresh meat and vegetables, salads and fresh fruit salads. Wine is also self service, you can fill up a carafe of red or white from a barrel. It's very handy if you have problems ordering off a menu in Italian!

For a cheap lunch with great views, climb the steps at the Vittoriano Museum Complex and eat at the cafeteria there.