It isn't true that something is necessarily worth what one pays for it. This assumption provides for a nice way to model economic transactions but it isn't always true. For instance, life saving heart surgery has a definite cost and it is worth far more to the recipient in some cases than what they pay for it. In the other direction I don't think that the cost of government services is worth what I pay for them since I think the U.S. government squanders far too much money on national defense.