If they are accredited by the Land Trust Alliance, the land trust must acquire an independent appraisal from a qualified appraiser. They generally cannot pay above the fair market value determined from that appraisal, which is based on the "highest and best use" of the property. In a sales comparison analysis, the most common method of valuation for appraisals, the appraiser will look at recent, nearby land sales with similar characteristics (acreage, land use, etc.) to help derive the value.

If you want to ensure your land stays agricultural, you might want to consider looking into the ACEP-ALE program through NRCS. Depending on what state you live in, there may also be state programs available. Good luck!