Right now there really is no way to.... There are instances where data is valued and where data is bought and sold. There, you can ask what the market value of that data is and how it's used, but often there is no sort of market value for that data. It's hard, because you don't know exactly who is using that data for what purpose, and how that data may be used later on when it's merged with other datasets. I really don't know the value of my data, and I don't know if I'm going to get a fair deal, much like the earlier example of getting a cup of coffee. I don't know if that's a fair exchange.
The other thing is that in a competitive market I would have alternatives and I could see how much they would pay me for my data. One thing is, how do you unilaterally assess the value of the data? Second, is there sufficient, robust competition so I can get a true market price for my data?