Costco's pizza works out pretty damn well on a just-get-some-calories-in-me-but-don't-make-me-cook basis, too, if you want something you can carry out. Two days of calories for $10.
I don't know what it is now, but Cici used to have $1.99 all you can eat pizza buffets on certain days of the week. This would have been in the mid 90s. Unbelievably (suspiciously?) cheap even for then.
Literally all you can eat + pasta. Dirt cheap. If you're the 'one-meal-a-day' type, is there a cheaper option?