This doesn't really work. How much would the game cost on release? What is the determining factor for what drives the prices down? If I own a game, and other people on my friend list also buy the game, does the price go down for buying it? If I buy it first, do I pay a larger price than all my friends who buy it after me? Do people get a check box for naughty or nice, then the game price is determined by that?
The system isn't broke because everyone has to pay the same for a game. The system is broke because certain companies charge $50-$60 for everything they make, no matter how shitty it is, no matter how much money they put into it.
Offering incentives to be an upstanding community member is one thing, but punishing people will make everyone's life harder. I don't want to have to worry about someone getting pissed because he invites me into a game and I decline repeatedly.
Also, what about people who aren't into playing online, how does that work? An earlier comment said then perhaps someone who doesn't use online would have to pay full price for the game, which is retarded. Why charge someone more for not using all the features of a game?
Edit: Encouraging people to play nice is good. Punishing people like the police for not being nice is not good. This will only create a more hostile play environment. Say a guy decides that he hates all the people who play a certain game, maybe it's worth him going online and streaming racist profanities for $100 extra. Valve, at that point, has no right to punish him, he is paying of the right to do that by paying a significantly extra amount of money to do it.
Edit2: The forced nicety will have the same negative effects of civil rights era laws. Forced integration strained relations of people who were different races. This won't make people be nice, it will simply give them more justification to do the things they do based on the extra money they payed to do it.