that's a tough one too answer, what kind of barn, does it come with a pig flow, manure easements, do you get the manure, condition, what kind of up keep did it have, room for future expansion?????    as far as the per pig space building cost. depending on building type, start with about $265 todays cost and deduct about 3% per year for the preceding years and that will get you pretty close.   that's for a curtain sided natural vent building, no land, well,or permitting.