Maybe you can try some of the on-line car databases.
f.e
http://www.dupontregistry.com
US Diablo prices seem to be between US$ 120K to 200K
Prices in Europe vary between Euro 70K for an early 2wd Diablo to 120K Euro and upwards for 6.0 and roadsters.
Of course a lot depends on milage and overall condition / service history.
So why sell the GT anyway and buy a 2wd instead ? Seems like a downgrade to me.