For a really cheap lunch, buy a hotdog at one of the carts. There are plenty of tables around to sit at.
For not so cheap, but quick, if you like hamburgers, go to Richter's Burgers located near the Graveyard Revue. They are not real cheap, but they are good. If you are not big eaters, you can share your order because the burger is a good size and is served with fries. I believe Richter's also has chicken sandwiches. Another restaurant for quick counter service is Louie's Pizza, also located near the Graveyard Revue. They offer, besides pizza, a panini (but my husband does not like it because they serve it cold), meatball sub, fettuccini Alfredo, and spaghetti. There is also the Monster Cafe (near Shrek and Twister) for chicken and pizza, and La Bamba restaurant (across from Mel's Diner which I do not recommend) for chicken and cheeseburgers. (Richter's is the best place for cheeseburgers.)
We try to do lunch at 11 am before the lunch crowd, but last Saturday we had to have lunch at 12:30. We went to Richter's and, although there was quite a long line, service was pretty quick. But still we prefer to beat the crowd, so we can relax while having lunch. Then back to the fun!