Property value question (lot of information in comment section)

Family has property where a gas processing plant is operating. When it was established over 30 years ago certain things were put into the lease, which included returning the land back to the way it was prior to the original lease. Learned this week that the company is trying to get away from leasing and wanting to potentially purchase. Tax records have it listed like it is farmland but they have several buildings and various other things on the property now. Am looking to find what would be an accurate value for the property in its current state, and what the costs could potentially be for the company to return it to its original state which would include removing buildings, gravel, concrete contamination, etc. Just wanting an idea of what would be appropriate for the 20 acres.

Should add that if they choose not to lease that is what has to be done in returning to the way it was before pipeline and buildings were placed on property.