While prices vary by region, I think you might be low. Take a look at Autotrader.com, Cars.com, and Edmunds.com to see what things are listed at in your area.

For example I found these on Autotrader zip 33301:

2006 4.8is for $22k
Cars for Sale: 2006 BMW X5 4.8is in Miami, FL 33154: Sport Utility Details - 369212748 - AutoTrader.com

2006 3.0i w/ salvage title for $15k
Cars for Sale: 2006 BMW X5 3.0i in Hollywood, FL 33023: Sport Utility Details - 358938574 - AutoTrader.com

2006 3.0i 65k miles for $14k
Cars for Sale: 2006 BMW X5 3.0i w/ Sport Package in Fort Lauderdale, FL 33338: Sport Utility Details - 377481693 - AutoTrader.com

__________________
2006 X5 4.4i
Sport package, summer package, cold weather package, DSP + Nav
Bought 2/14 with 73k miles