Cambridge has many cheap eats for travelers on a budget:

Harvard Square

• The Garage - This mini-mall is filled with pizza, ice cream, sandwiches and other basics
• Pinocchio's Pizza - Serving slices for generations
  
• The Hong Kong - Notoriously greasy Chinese food, but inexpensive.

        (note: despite it's appearance as a diner/shack type place, Mr. Bartley's Burgers is actually quite expensive. Potentially worth the price, but not for the budget diner.)

Central Square

• Middle East - Family-owned restaurant with 3 nightclubs for live music as well
• Picante - Take-out or eat-in Mexican
• HiFi Pizza - Also noted for its late closing hour
  

Porter Square

• Boca Grande - Gigantic cheap burritos. Also in Kendall Square
• Porter Exchange - A collection of Japanese restaurants - sushi, noodle bars, bakeries and more.
  

Inman Square

• Bukowski's Tavern - Burgers and dozens and dozens of beer choices
  
• Punjabi Dhabi - Indian food served buffet style
• S&S Cafeteria - Same as you had as a kid