My first thought here was that this is an intriguing idea worthy of the genius at Valve, but when they actually playd it out, it would inevitable be far too open to breaking. Then in the discussion, I realized that this...could actually work. It would require tweaking and iterating to a degree practically unprecedented, because even tiny flaws in the system could lead to massive monetary loss and consumer dissatisfaction, but there are principles here that could be incredibly effective. But I have a few suggestiong for Valve.
1) Punishment is less effective then re-enforcement, and if you use bad behavior as a way to generate more revenue, then you will only have frustrated players. To do this, you will have to stick to rewards for positive behavior except in the most extreme, destructive behavior present in only the absolute tiniest minority of consumers. Then you can charge a small markup, but even a tiny markup for the worst behavior should discourage the bad players.
2)Tone it down a bit. Huge changes to price is asking for an unsustainable business model, and paying players should be limited to players who add content that increase your overall profit. Remember that Achievements, effectively tiny icons in a list that don't change anything in game, act as huge motivators.
3) You are a business, so reward behavior that makes you money. For example, if a player recommends a game he already owns to another player, and that player buys that game, that is worth rewarding. If a person has bought all your games, then reward them for being a repeat customer. Inevitably people will try to cheat the system by discussing with there friends, and making recommendations for games there friends will play anyways. But if this happens, it means people are trying to maximize there strategies by buying more games, so you still win. Make sure that no matter what a player does you are still profitable, and there shouldn't be a problem. Consider adding a time frame, so you get better rewards the faster you buy games. This almost slides into insideous, but people may very well go on buying sprees to maximize there rewards.
4) Consider including non-monetary rewards like hats or aesthetic content as an incentive, and consider integrating achievements into this system: Achievements are powerful motivators as useless decoration, and they have huge untapped potential. If people are buying games so they can get achievements to contribute to some meta-game, you are going to profit greatly.
