I would think if someone is offering you a set price then it must be worth 4 times what they are offering
There's pros and cons for selling. I can assure you, you will make more money in the long run. But I once had a landowner tell me "why do people take the lump sum when they win the lottery even when the lottery is a 40 year guarantee when royalties are not?" That kind of opened my eyes. Here's a breakdown that a buyer presented to a landowner I'm working with.
Leased for $4000/acre on a good lease with 18% royalty. Technically he gave up 82% of the future profits for $4000/acre. Obviously he doesn't have his own oil and gas rig to produce his own rights but he felt $4000/acre on a lease was a GREAT deal. He was offered $8,000/acre to sell his rights. If you break it down on a per percentage point basis then his lease paid him $48.70 per percentage point he gave up. If he sold at $8000/acre for his remaining 18% then he would get $444 per percentage point. I can assure you the gas company isn't going to just leave oil and gas under his property and walk away so basically by leasing he gave away 82% of the profit that he will never see. Looking at it that way definitely opened my eyes that lease prices are extremely low.
There's a million ways to look at things but I just thought I would share that.