I’ll be surprised if anyone can tell you a “market rate” for a surface waiver.
From experience in other Texas counties it generally comes down to what the particular mineral owner (or possibly multiple owners) you are dealing with are willing agree to, and generally there is nothing that would force them to agree if they aren’t willing to waive their rights.
The size of the overall mineral interest and what portion of that interest you need a waiver on could be important, as well as whether those minerals are currently under O&G lease, or in production.
For example, a single mineral owner who holds 100% of the mineral interest in a 640 acre section where you only need a release covering a specific 10 acre part would probably be less concerned about granting a waiver than the owner of a 50 acres of minerals where you are needing a release that will cover all of it.
Your situation may be different but if you’ll go to the Looking Glass feature at the top of the page, next to +New Topic, and put in a search term like “Reeves County surface waiver for solar panels” you may find some previous discussions that may help.