It's not too bad...that's about average. I've seen extremes sell in that range over the past year. I knew someone that bought a 2006 extreme last year for $52k. I knew another person that bought a 2006 extreme for $49k. So I'd say that's about the right depreciation for a 1 year old extreme.
I'd try and get them down a little more just to see if you could do it. Not everyone has the money for a $52k used car (nor do a lot of people know what a saleen extreme is).