As above, I see them going (4.4's) for anywhere from $5000 to $10000 depending on mileage, cleanliness, etc. I see really clean examples priced by dealers @ $10000 - $13000.

I'd sell it privately on Craigslist or Autotrader. If it takes 5 or 6 months to sell then so be it. If you are in a hurry then a trade-in will probably be your best bet, but don't expect more than $5000-$7000 depending on what you are buying.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)