I would say, really the eye of the buyer. To me, unless it is a late model 325iS/325iX, or Mtech 2 convertible or an e30 M3, they are not worth very much at all. I don't value small tail lights, metal bumpered, non "i" e30's much.
I can see someone wanting it for the clean shell to go on with a crazy engine swap/project. That means, they wouldn't want to pay much for the car since they don't need anything other than the shell.
I would shoot for 2500 and work my way down. ?
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--'00 BMW ///M5 .:Jet BLK/BLK:. Individual-1 of 1 - Style 65 Wheels; 9.5" Square Set up;275 Tires;10mm/3mm Spacers; Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'04 Volvo S60R/6spd.:Nautical Blue/Gobi&Black
'00 BMW ///M5 .:Avus Blue/Grey/Black:. 1 of 8 SOLD
'88 BMW 635CSi Blue/Tan Sport
'81 MB 500 SEC Euro Koenig