The SL Market Letter actually tracks the SELLING prices of cars, not the ASKING prices, when available. They have been publishing their Market Letter for quite some time and have lots of historical data. They do price trends. I would visit their site and subscribe. The also collect For Sale ads from many sources and compile them into their publication. They also make sure they don't run more than two issues in their publication, so you don't have old stale ads. You might want to pay the consulting fee to speak with Mr. Olsen, he's very knowledgable. We did an article on him in the first Pagoda World.
Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420