There can be a lot of factors that go into this phenomenon, from the player count on a given system and slight alterations in item availability. There's also no trading between different versions of the game, so players can't try to get a good deal on one platform and then sell it for more on another. It's also worth noting that you won't find that you can consistently rely on one system to have on average cheaper items than another, as it varies over time and on an item-by-item basis.