Char-grill is a popular local burger joint; done up like a throwback to the 50's hamburger stands, it's got a classic appeal and really good, simple menu.
My daugher just discovered the "Cook Out" resturant, a regional chain out of Greensboro. Excellent food, very economical, great milkshakes. Here is a review and links to the locations and a menu: http://raleighdurham.about.com/od/diningandnightlife/gr/cookout-review.htm
If you like good BBQ for cheap, try Danny's.... http://www.dannysbarbque.com/
Jack's Seafood & BBQ is an offshoot of fish monger business next door... It's a hole in the wall place, but has great southern food. Yams, Fried Okra, Turnip Greens, Hush Puppies, Sweet Tea... Oh yeah, and the fish is pretty good too!