Golden Corral?
Seriously, for $20 a person you will be seriously limited. By the time you back out $4 for tax and tip, it's going to be pretty hard to feed people at anyplace other than Chinese, Perkins, BBQ, etc. for $16--I'm assuming you'll want to budget a cake and beverages into that as well. Maybe Macaroni Grill or someplace like that? Even there, $20 isn't going to cover appetizer, entree, dessert (cake) and soda/tea/coffee.
I can't think of a single place on property other than food courts and fast food that will stay in your budget.
Maybe check with some hotels and see if you can do "heavy appetizers" and a two hour reception to stay in that budget. That's probably a lot more doable.
Another option might be to get married in the morning and have a wedding brunch someplace--that should be doable in your price range.
Good luck!
Anne