Values have gone up ... Just not as much as you want
I think values have gone up considerably. Five years ago a clean CSL could be had for $50-60k, and a nice Csi for $30k. In the last 2-3 years I've seen few coupes sell in the $40k-70k range. That's pretty good for a GT car of the period. Ferrari 308's and 328's sell for no more, and the four seaters are dirt cheap. One of my favorite p-cars is the 944turbo, and those are only in the teens. 1975-80 911's can be found all day for below $20k. The Merc SL's of the period are worth far less than an E9 in comparable condition.
Let's be honest, E9's are fairly appreciated and valued for what they are. They are simply a mass produced GT car that is not the
-) fastest ( Pantera, P-turbo, 911s, etc)
-) rarest
-) prettiest (compare it to a 70's Dino, Miura, 512BB)
-) winning-est ( eg not a p-car). They aren't eligible for most vintage racing, CSL's excepted.
-) snottiest (AM)
I even think E30 M3 sport evo's will eventually surpass E9 values because of their importance in racing. In a year we'll be able to import the Sport Evo from the uk and Europe. The e30 has much more historic significance for BMW ... And like the E9 it also seats four for Sunday brunch.
My advice, stop worrying about it and enjoy these wonderfully practical classics.
If investment value is your priority, I'd buy a couple early 230/250 Merc Pagodas or an M3 Sport Evo and stick them in the barn.
Sincerely, and sadly sans E9' at the moment...
John