Prior to purchasing my 06 4.8is, I looked at ebay's sold listings to determine actual selling price. You may want to check that out. From what I saw there last August-October, most with around 100K miles or near, sold prices ranged from mid-$12's to high $13's, including the rarer Imola red.

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2006 E53 4.8is
1998 E39 528i
1998 Corvette Indy Pace Car
2005 Dodge Ram 2500 4x4