As Martha mentions, the value of your minerals is very dependent on the location so the offer could be low, middle of the road, or high for the specific area.
In terms of how to decide if you should sell, that is usually a personal decision based on your situation and financial goals. If you are willing to do some homework and basic education, it may be worthwhile to hold on to your minerals. If you have questions in this area I would suggest talking with your financial advisor and/or accountant to understand the implications of selling. Another thing to consider is if you would like the up-front cash now but would like to enjoy the potential up-side is you could always sell just a portion of your minerals. Just because a company makes an offer for 100% of your interest, you could always counter with a smaller percentage.
Just keep in mind that this is a business negotiation so pretty much anything is negotiable (similar to buying or selling a house).