If its in great condition, the NADA classic car value is $7525. Mint show room condition like you would see at a Barrett/Jackson auction, would be $15,840.
If it really only has 40K on it and is in good condition, his price is better than fair. The price fluctuates in different parts of the country, depending on area and demand for the vehicle, But, the price I listed is a nation-wide average.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...