Coupe prices
If I buy a car and it depreciates less than $1000/yr I am very happy. This has been the case with most of the cars I have owned (if not purchased new - which I try to avoid), including my e9 of four years.
I agree with the previous comments, specifically with the mistaken feeling that the $5-$10K e9 only needs a little attention to make it excellent, but as mentioned, the rust issues are present in most pre1975 imports. I agree that our site, arguably the best reference for e9s, should spell out the very high cost of body parts and rust repair while at the same time mentioning the reasonable engine and transmission components.
Are BMW collectable? Well, with the exception of the 507, they are not and will not be in the class of the Ferraris and Bugattis, but they should be in Jaguar, Mercedes, and Porsche territory. Keep in mind that the highest priced collectibles are often convertibles; in the 1970s BMW was not making convertibles. Just check the difference in price between an XKE coupe (which I like) and the convertible (2-3X more for similar condition). There is the cute factor and the cult factor for VWs. When I see Isettas going for $45K. . . But folks routinely pay $45K for the abominable Amphibicar - neither a good car, nor a good boat and almost as ugly as a Pontiac Aztec (RIP).
I think the increased interest in the CSLs will help the CS and CSis.
My take on collectible BMWs through about 2002 (I refuse to list the cute Isetta because it is not fun to drive):
328 - definitely
507 - the most desirable, now a million dollar car
M1 - definite, prices rising to $150K+ range
CS, CSi, CSL - yes, but see previous thread
2002 tii - yes, now $15-$25K for a nice sorted rust free example
e30 M3 - yes, $25-35K for nice example (still not more than original purchase price in 1988-1991)
e24 M6 or Euro M635 - yes, but prices still falling, albeit slowly, $15-25K for a nice car that was $60K new in 1987-88
Z8 - definitely collectible, did not suffer much depreciation - now are stable and typically $100-130K for a nice car that was $135K new in 2000-2002.
I agree with Murray, in the next several years, excellent coupes will be in the $25-50K range (or may be now), but don't count on six figures for nonCSLs. From the early 1970s, the Mercedes 280SL has done very well for a car with large production numbers, but then again it is a convertible.
And like Murray, take good care of your e9, but drive it a lot and enjoy it and be glad it is not depreciating like a new 6 series (like a rock).