When selling farmland, let's say an 80 acre parcel, does the purchaser pay only for the tillable acres, and the nontillable are included in the price, or are the nontillable priced at less? Looking at 80 acres, with only 77 tillable - offer is based only on the 77.
All land has value including non tillable acreage or scrub land. Any sales contract involving the transfer of real estate should stipulate what the buyer and seller are actually agreeing to buy and sell.
Your questions seems to be how to estimate the value of a particular property if only 96 percent is tillable. You can look at recent sales of like properties and determine an appropriate value for the tillable as well as untillable land. You might also look at potential use for the untillable land. Is if it is a fresh water spring does that add value? Is there any potential to convert to the untillable land to another use, mineral rights, could it become tillable? Ask the seller if they would consider a partial sale of just the 77 acres if that is all you wish to buy.
Remember all things are negotiable. Just try to make your offer a Win Win transaction and you will have a good chance of putting together a fair deal.