One (admittedly theoretical) way to get companies to reveal the cost is to tax them based on the self-declared value of the data. The lower value they declare,

One (admittedly theoretical) way to get companies to reveal the cost is to tax them based on the self-declared value of the data. The lower value they declare, the less taxes they pay, but the cheaper they have to give it away. One could do this for either personal requests, or even more radical, force the companies to sell the data, at the delcared price, to competitors. I think this is pretty much suggested by Eric Posner in his book Radical Markets.