Start here to compare a few examples. I've already done a lot of the heavy lifting. I built a spreadsheet that has every E39 M5 (currently 100) for sale across the country under $20k. It includes prices, miles, ownership records and more. Definitely join up at M5 Board too. Tons of information.

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His: 2006 M5
Hers: 2005 545i

Selling: 2004 X5