Not only are used copies of Final Fantasy XIII not worth very much money, new copies of the game are cheap, cheap, cheap. Great news if you haven't bought the game.
The PS3 game went on sale on December 17 for ¥9240 ($116). Shops have been buying back the game for ¥1500 ($19).
Retailer Sofmap in Tokyo's Akihabara has stacks and stacks of Final Fantasy XIII. Yours for ¥3980 ($50), but this is a limited time sale promotion. To point to this as a sign that FFXIII was some sort of failure in Japan is a mistake. It's not that at all.
However, it does show that retailers might have ordered too many copies of the game and are trying to unload copies. Bad for them, but good thing for consumers! If you haven't already, go pick up your copy at 59 per cent off. Do it.