The only rule of thumb on pricing mineral interests I've ever seen was to take about 2-3 times the annual production. That naturally assumes there is production, and also that you know for certain no further oil/gas deposits are underneath the surface. Which you naturally can never know.
After many years of reading about such things, I've come to the conclusion that a mineral interest is worth what you get for it. And that it is, as a rule, not a good idea to sell one unless you are in dire need of cash right now and thus must sell. Otherwise you should think long-term, and that means not only you yourself, but also your descendents. Mineral interests are not short-term investments, and it's impossible to set a price on them since there's no market like there is for stocks and bonds.
There's one more consideration in your case: if the tracts are already leased, they're less interesting to a potential buyer because he will be bound into a lease he may not like, and will not have the chance to lease the interest himself, immediately gaining a bonus that might repay some or even most of his purchase price.