moped, the short answer is that they are ripping you off.. the...

moped, the short answer is that they are ripping you off.. the long answer is that you are trading a derivative product where the prices that they off as a market maker are based on (1) what they can hedge that product for in the real market (2) the bets of other players in that mm's pool where the mm plays the role of the bank and so always needs to make a profit and (3) things like interest rates, volatility, risk, etc etc.. the mm always calculates these factors and ensures that they are not bearing the cost of those risks so pass those risks on to you as spreads and discounts (or premiums) to the underlying physical commodity (that is spi or xjo). The mm calculations are usually generously weighted to the mm. I like the first answer best. cheers!