It wouldn't be so much that you would 'charge' for your data, it would be a bidding system where companies that want your data would offer you a price (and you could see what they were going to do with it before making a decision as to whether to sell to them or not). That way the user can choose to sell to the highest bidder, sell to the bidder that has the most ethical use of the data etc. In all aspects, the user should be in full control of their data and they should clearly be able to see what it is worth. Much like shops have to advertise EXACTLY what a product costs when it is money that is changing hands it should be the same for personal data.