How is the mineral rights value determined on producing wells?

I am trying to determine the value of the mineral rights for two producing wells.

Our last buy offer was 2010 for one well and 2011 for the other well. I think the land is worth more than I was offered, but not sure how to determine this.

I could value the offer plus 10%, or take the royalties per year and extend them for 5 years.

Not sure which way to go.

To start with, maybe I should brush up on my English:)