around $15K for a low mileage '04. the '05 and '06 go for a bit more.
www.ls1gto.com is a good site to go on, if you can ignore the people that consider tinting your windows, or plastic "billet", a major modification
there is lots of great info on there
BTW, I own a 2004 GTO that I have done a lot of work to, so I have a bit of an insider's perspective
check out the link, and you can see some of the stuff I've done. I've done everything that you see on my own, except Pete at Comp Spec did all the paintwork. I coludn't be more pleased with his work, and that's why when I get the money to do my '69 Camaro over, he will be doing that also.
http://s496.photobucket.com/albums/r...20GTO/?start=0