Robbin,
That is a big jump in the offers. You mentioned that your window to decide is short. I assume that is based on conditions described in the offer letter? If a mineral buyer wants your property this week, they’ll want your property next week too, so I think taking your time is important.
I’d recommend getting competing offers. Unsolicited purchase offers often do not reflect the actual value of the minerals. I’d also recommend that you have someone evaluate the offer letter and any other instruments you’ve received to make sure it isn’t a bait and switch.
As for a mineral appraiser, the monthly check is one component that they would look at, but they would also look at the future development prospects based on what has been proven in the area.
Best of luck!