The Catwoman LEGO Minifig only appears in the LEGO Batman set The Batman Dragster: Catwoman Pursuit 7779. The LEGO Catwoman mini figure is 2" tall and comes with a specially molded "mask" and a unique whip. This is a particularly hard to find LEGO figure and the prices you will see it selling for reflect that. It is not common for a mini figure to sell for $50 regularly but the Catwoman figure is unique because it only comes in one LEGO set and that set is discontinued so it is very hard to find.
Check out the eBay auctions below and you might find one at a decent price.
Catwoman is probably Batman's favorite enemy and also one of the favorites for many fans of the Batman movies, cartoons, and comics.