I went to allears and checked on prices, here's what I found.
MK - Tony's has several entrees for around $20; Plaza is mostly sandwiches, but is definitely one of the cheapest entrees you'll find at a TS for dinner;all other TS here are closer to $30+
MGM - 50's PT and Sci-Fi Dine-In, all entrees less than $20;Mama Melrose, most are less than $20
AK - I wouldn't recommend TS here even if it's cheap.
Epcot - Biergarten, Nine Dragons, Rose and Crown have several or all entrees for $20 and less
Resorts and DTD - there are too many restaurants for me to look up every one right now.

But here are a few I found for you
Big River Grille and Brewing Works, Spoodles, Grand Floridian Cafe, Olivia's, Kona Cafe, Cap'n Jacks, Raglan Road
Check out the menus and decide the kind of food you might want. I would vote for Kona or Raglan Road based on things I have heard from these restaurants. Just some info. for you.