Are you sure you are being offered an amount just for the surface? If your surface and minerals are not severed, then they may be offering for both and if you want to keep your minerals, you would need to get some legal help to sever the minerals properly should you decide to sell. In addition to what Tim said about okcountyrecords.com, go to your local newspaper or real estate groups in the area and see what the going rates are for surface in the area. Always ask yourself, why do they want my acreage and what do they know that I do not know and what do I need to find out about the going rates in the area. If you do decide to sell, never hand over a deed without getting paid at the same time, not in some future time. Getting legal help for legal matters is usually a wise investment.