I'd imagine it's gonna be a used top end card from a few years back, as opposed to anything current gen, but how far back you'll need to go to get below $100, I don't know.
A 550Ti will support everything but TXAA which most cards don't support anyways.
I bet if you're lucky you'll pick up a 560 for that price though, 550Tis went for $100 new a lot after rebate.
Actually, Samjc3 has a 560Ti for sale on MR right now for under $100 which is a great deal.
This one is just a tad bit over $100, but it's amazing. It works really well, it's extremely quiet, and it can run high end games on max settings without screwing itself (that depends on your ram also, but still)