It's a rare car, but also has limited demand. The last M5 that actually sold on Ebay went for $24,902. There haven't been a lot of them sold at auction. One sold wholesale here in Phoenix for $26250 with 48K miles in February.
Personally, I like the sound of this on on Ebay:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=130207904298
It's owned by the service manager of a BMW dealership. It's a 'no reserve' auction currently at $23,200. Could be a good deal if it doesn't go much higher.