Lucky cats are a traditional Japanese good-luck charm. They're called Maneki-neko. As far as I know, they became popular sometime during the Edo Period (that's from the seventeenth to the middle of the nineteenth century). Many shops in Japan have them in their window, with one paw raised as if to invite customers in. Since they are said to bring good fortune, quite a few Japanese have one in their home. (Besides, there are a lot of cat lovers out there!)
There are several legends explaining why the cat symbolizes luck. According to one, a monk's pet cat beckoned him away from a tree under which he was standing in a rain storm; soon after, lightning struck the tree. According to another, a woman's pet cat began tugging on her kimono. A man thought the cat was bewitched, and cut off its head. The head, in flying off, killed a snake which was about to bite the woman.
White cats represent purity, while black cats are considered lucky (quite the opposite in the US.) There are also some multi-colored cats available. The cats usually come with a red collar and bell. One paw is always raised, although on the US market the paw generally faces the cat, while those available in Japan have the paw facing the viewer.
And the price? They run anywhere from a couple of bucks for a key-chain version, to hundreds of dollars for the gold and silver version. You can pick them up in most import stores for, say five to ten dollars.