Kokopelli kind of have the right idea with their BluFilter P2P add-on: artists set the prices for their tracks, users pay the set price plus a bit extra for the distributor. Where they go wrong is not paying anything to the supplier - instead all the extra money gets siphoned off by the P2P network owners. This means a) all the money leaves the system and b) there's no incentive for participants to upload quality tracks to the network.