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There's quite a few algorithms out there for economic simulations like that, ranging from simple "the more you buy, the more it cost; the more you sell, the less it costs" to complex ones that modify based on any number of point-in-time history elements, with older history items having progressively less of an impact until they simply disappear. I think for a game like this, the economy portion of it is not a major game component, so a simpler approach should be suitable: If the player buys, that would drive the price up. There is no selling done of anything but energy to counter that, though.