If you find yourself at Cosmopolitan Hotel your options for affordable are pretty good with Holstein's, D.O.C.G., Secret Pizza and the reverse Happy Hour at Comme Ca. A couple of tacos and a beer at China Poblano might fit you under the $20 cheap food ceiling.
Everyone of these restaurants has hidden gems on the menu just remember that alcoholic drinks are the killer of the cheap meal. Actually, have free water and your bill stays low. Go downstairs and put a few bucks into a slot machine right as the waitress walks by and you can get a free drink instead.
Looking for cheap food in Las Vegas?