For 100$ combined you can't get two cards for crossfire.
I mean, you can, but they will be very low end cards that probably were never meant to be crossfired.
Take your 100 bucks and buy a 4770, or a 4850 (or whatever Nvidia equivalent there is) and enjoy knowing that those cards all beat whatever crossfire setup you could muster with 100 bucks.
I would say that the cheapest card that you should crossfire with is a 4770 with another 4770, other than that a single card is your best option by far.