Added cost and resale multipliers to fix issue #117

I've added more multipliers (2, 4, 8, 16) that increase the cost of an item depending on the amount (10, 100, 1000, 10000) owned.

I've added the same multipliers to the resale function to maintain the games balance.

This addition should help balance the game economy during late game-play, the multipliers chosen are fairly arbitrary.