I don't know about the older SDs, but the price does seem to be moving. A friend of mine (my indy) just sold his 84 300D (about 130K miles, I think) for $12K. This car was a nice driver, but far from showroom. He had it listed for $12.5 (someone here linked it and said he was dreaming), and got an offer for $12 within a week. It was in pretty good cosmetic shape (I've driven it while he worked on mine), he had the paint spruced up and replaced the grill/headlights, the interior was about like mine (maybe 9/10), the engine/tranny feels strong but I always thought the steering was a tad loose.

As I recall, he paid about $5K for it a couple of years ago (it had a small dent in the door) and probably put $3-4K (and some labor) into it to fix the body and a few other little nits.

It appears that some folks are willing to pay a premium for relatively low mileage cars.