I'm in a similar position but looking at buying a used 2006. Locally I found one for $28.5K with 26K miles. That is without negotiating.

The best way to figure out the true value is pulling the Manheim values. Those are the auction prices. There are many used X5s on lots, so the auction is the only place for them to go.

I'm going to do some digging and try to get the auction prices. For a 2005 with low miles, I'd say something around 26K is fair under the current market.

Prices are falling like crazy, I was going to buy a new X5 but decided I could get a warranty and low miles at just about half the cost. I've owned the E53 and loved it.