The Osaka SuperPotato has a stack of them for 500yen each though they are kind of buried and really hard to find without digging. I've seen them cheaper in a lot of the local non-chain stores too. I don't think there's much demand for that game (or most FX games in general). Even the big 4 (Zenki, Zeroigar, CCK, TMCP) seem to be priced for less recently. Lately Zeroigar seems to be the most expensive here, but the price is still much lower then the typical "rare game" prices for the popular systems.