Marginal utility. If the marginal utility per dollar is higher for one good than the other, that means for every dollar you spend on that one good, you are getting more utility (benefit or satisfaction). For example, let's say you get 3 MU from one burger, and 1 MU from an order of fries, and the burger costs $6 and the fries costs $1. MU / P is the equation that tells you marginal utility per dollar, so 3 / $6 = 0.5, and 1 / $1 = 1. The fries are the better buy because their marginal utility per dollar is twice as high as the hamburger's.