There's an interesting piece in the New York Times auto section about a father who made a deal with his teenage son to buy him a replica kit of Shelby 427 Cobra (read: rare, super-expensive 60s sports car) if he maintained an honor roll position in high school through graduation. (The father introduces his son as "underachieving.")
I never knew of an example quite as extreme as this (this car is many tens of thousands of dollars), but I do remember miniature versions of this scenario in high school. In this case, I like particularly how it appears that the incentive was offered rather thoughtlessly by Dad and not spoken of before graduation, but once the son had his diploma, he expected the bargain to be honored. There's interesting tension there; I wish I could hear this from the son's POV.