Go to county clerk's office. Usually have computers there. They'll have their appraised amount values. Plus mortgaged amounts. They do not show purchase prices.
Personally, I'd look up who purchased the lot and get their address. Then I'd personally visit if possible. Ask what they paid. I'd have a letter for them to sign regarding purchase price, address, and their name and signature. This is just me.
However, I'm a realtor.