A new Porsche has been a poor deal for a long time now. Way back when a new 911 was the same dough as a Corvette, it wasn't too bad. But, the 80's Yuppies turning the brand into a serious status symbol has killed any value in buying new.
Used Porsches are the only way to go for an enthusiast driver looking to avoid the serious initial depreciation that comes with any Porsche.
I know a fellow that picked up a hardly used (~8000km) Viper (I think an 06 or 07) for under $50K. Not my cuppa, but that is a LOT of go for the dough. Of course, the dealer was asking some outrageous number for it, but he went in armed with cash and a top-out number and no budging.
Personally, I think the GT500 is a lot of bang for the buck, but I think the plain old "only" 400hp GT is better. $20K is a lot of money.