There are two different and very distinct questions there. Please separate them.
First is: how much licensing royalties have actually been paid.
Second is: If you were trying to understand the economic value of a particular patent, how would you go about it?
For the first, I can't imagine any honest way of doing it. As a dishonest way, you could make an offer to buy it from the owner, and find out what they demand. Or do the reverse and call the licensing party and pretend that you have a substitute for that invention.
For the second: I was actually the engineer interviewer at Google, at a time when they were hiring people to acquire patents. I asked a variant of your question to the candidates, and as far as I can remember, everyone had a different answer. "$300,000 per patent" is a rule of thumb. They are worth much less than you probably think.