A car is only worth what somebody is willing to pay. If you're the seller and think it's worth XX dollars, then by all means try and sell it for that. If nobody buys it, then either you're wrong or you haven't turned over the right rocks.
If you're the buyer, then remember that the most expensive mercedes is the one that you can buy for a song. Nothing like buying a 230SL for a few grand and then investing 250 hours of labor getting fresh steel and paint followed by g-d knows how many hours dealing with everything else.
Find a car, make an offer based on what you can afford and what seems like a reasonable number to both you and anybody else that shares your expenses and if the seller says "no", then perhaps you've offered too little. Then again, perhaps the seller should read this post.
One thing is sure. There are more cars out there than you can imagine, presuming you turn over the right rocks. As a buyer, I would strongly suggest investing in a year's subscription to the SL Market letter compiled by John Olson.
Seeing the advertisements for the half million dollar gullwings makes the price of a 250SL seem rather reasonable. Seeing 30 cars for sale gives you a better idea of what's up.
-CTH
PS. Cool, this is my W113'th post!