Shelby County TX gas oil mineral rights lease

I have been contacted by a landman regarding mineral rights from my great grandfather that has been handed down to me and my siblings that he signed 1931. What would the going rate be per acre ? We have 112 acres in shelby county TX. Is a $50.00 signing bonus fair to sign the oil gas mineral rights lease? .How many of my siblings have to sign to make the deal. And what about all other hydro carbons and petroleum products not mentioned in the lease.