I have seen some '91 850's in the $10K range. The really low priced one usually have problems though.
If you have a good set of tools and know how to use them some may be bargains.
If you have to go to the $tealer for everything you must be prepared to mortgage something if it has serious problems. BMW still thinks of them as $100,000 cars.
Some day I'll trip over a poor abused 850 somewhere and won't be able to resist the challenge.