Sale value and rental value are almost unrelated, in this day of insane competition to rent more land from owners who don't want to sell. Its one thing to pay too much rent while commodity prices are high, knowing you can bail out if prices fall; but most will think twice before paying an outrageous purcahse price, because you're locked in for life.
I agree that the lawyer doesn't seem to "get it" (or maybe he's in cahoots with someone). Get a smarter one.
Only fair way is get a fair market value appraisal, by an agricultural appraiser. Around here, Farm Credit has ag appraisers, and they hire out for private jobs as well. DON'T get a residential appraiser just because he's cheap- this is no place to economize, as hundreds of thousands of $$ are at stake. You need someone who actually knows what he is doing.