There is an article in the latest Unique Cars magazine about these cars. With respect to prices it is all fairly subjective, that is, it all depends upon what you want, how much you are prepared to pay and what condition the car is in.
As a fairly rough indicator, see the following;
XA GT sedan approx $10,000 - $20,000
XA GT coupe approx $15,000 - $25,000
RPO's, ????????? I believe you may be up for $40,000 - $60,000 for a coupe, yes I know, this sounds excesive. I am only going on what I have been told from a fellow that has an RPO sedan and what another bloke was prepared to pay for a coupe. For his sedan he wants a shit load more than the standard GT.
I recently bought an XB GT sedan in need of restoring for $2,500. It's a real POS but once again, it all depends on what you are prepared to pay, how much you want it and if you can find it, its real value???
(heaps of good info on this one)
Hope this helps. Don't take my word only for the pricing, talk to as many people as you can. Another person to ask is at this address;