I think you are getting hung up on a detail. My 190E 2.3-16, a mere 4 cylinder 190E cost $47,000 fully equipped in 1986! I think the point DR.DIESEL made was the price in actual dollars for a similar car has not changed in more than ten years, and in most cases has come down about 10% or more. If you invested that money for the last 10 or 15 years, it would surely be worth a hell of lot more today, even considering the present market conditions. And other vehicles, like Honda Accords and Nissan's and Toyota's similar offerings, which used to be under $7,000 fifteen years ago are now at the $25,000 and higher level.
If you take a different perspective, Mercedes-Benz is now building cars to challenge the low priced imports as well as their luxury offerings, on price as well as deliver all that is Mercedes. It is proving perhaps to be a little more difficult than it may have originally seemed, but it is good to hear that the new cars are coming in better sorted out and using better quality materials in response to valid criticisms like those from yeldogt and others here.
So, rather than wonder what the actual cost of an actual car might have been with specific set of options (because what it sold for is likely not available, and we all know that in the days of those list prices there were much better discounts provided to buyers than today), focus on the point. Had you invested the money for that SL in the mid 80's in say, Microsoft, you could buy a dozen or more of today's SL55's. The new products are offering a lot more machine, with all kinds of electronics, for much less actual money.
If you want to give the Japanese automakers credit, it is for pushing all this electronic stuff on us. They drove the thrust to make it all standard stuff for a luxury car, and we accepted, even endorsed the idea. Luxury no longer means "lasts forever" or "bulletproof mechanical design and manufacturing" or "exceptional handling" or any other aspect of what a great car was before it became a place to stash electrical and electronic doodads that you couldn't sell by themselves. Electric seats and windows drive me nuts. They should not be standard, as you should be forced to understand how much of the car's cost and then price is made up of this junk. When I bought my Euro cars, I had the option price lists for all this stuff. Electric seats cost and extra $1700 each in 1985. The automatic climate control system you all hate to repair cost $3400 extra, over the manual air conditioning system. Who the hell needs this stuff? If I am cold, I can push a lever in the "not so cold" direction, or, if I am too close to the dashboard I can manually change my seating position. No one would buy this junk if they had to pay for it item by item. I am going to stop before I get on a rant and set a new record for the length of these things.
The bottom line is if you could just buy the car, an SL55 would probably cost under 60,000. But because of us, you can't. Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)