Question about value of non producing mineral rights acres

I would appreciate it if someone could tell me the value of non- producing severed mineral rights acres in Washington County. I own 923 non producing acres and 240 producing I have been told by a company that approached me wanting to buy that the non-producing is valued low at 40$ an acre. That seems very low as much of the non producing i own is in the same area as the producing which nets to me $800.00 a month from Stelbar oil corp. Any help would be appreciated on this. thanks

I do no know the value of your minerals regarding an outright purchase but I have been offered $20 a net mineral acre for a 5 year lease with a pooled mineral rights clause. Given that; $40 seems low.