Hah. Your software for assessing "the proper prices of things" is separate from your preferences. In a nearby hypothetical world where $5 were the normal price for M&Ms, I think you would've bought them at that price. Because in our world, when the universe took $4 away from you as a fait accompli but allowed you to enjoy the M&Ms for $1 in return, you liked it! In effect, you were deprived of the chance to notice the "unfairly high" price of $5, so the blame/propriety circuit was stopped from interfering with your actual preference =) If the sole bill in your wallet had been a $100 but then "luckily" morphed into $1, I guess you wouldn't have been as happy!