funny you posted this, i am looking at an 06. i think with so many on the lots they are going for the low side of the range. not sure what headers and all that add to the equation, but people on the GTO forums are talking about getting new 06s in teh high twenties.
$25K sounds pretty good with some aftermarket mods. I read in a mag somewhere that after market mods and other 'improvemnets' should only add 10% of the cost of the parts. Is this a private seller or a dealership? If it's a private seller, see if you can get them to go lower since a dealership will give you crap if you get some warrenty work done with those modifications.
18" Enkei's & Kumho Ecsta's-------UNICHIP Eibach Sport springs----------Morroso oil pan Injen CAI & Oil cap----------Stage 2 FI cams Custom Exhaust-----------TRD Supercharger AEM alt. pulley -------JSP Carbon Fiber hood 380cc injectors----DC Sports ceramic header