The ONLY official info. from Ford has been that the V6 will start under $20K and the GT will start out around $25K. But with the 1st official production 2005 Mustang rolling off the assembly line tomorrow (Tues Sept 7) I'd guess that Ford will give us the prices very soon.
Power wise, the 05 GT V8 will be almost as good as the 99 - 01 Cobra and 03 - 04 Mach 1 engines, but it will run on regular gas and weigh about 60 lb. less.