IMO, People are buying FSU often because they are influenced by FSU. This way or another.
I don't think here is as much services for IIa/IIIa as for M2/M3. Market does reflect it with cheap, dead Contax bodies with famous Zeiss bumps on the back. IIa not working for under 200$ is common on ebay. IIIa isn't much different if not less for camera which needs CLA forty years "already", somewhere...
M2/M3, first you have to find them in dead condition, because most of them are looked after and they are in fact more expensive than IIa/IIIa.
IIa/IIIa have basically two lenses with as primitive as in FSU viewfinder. 50/2 and 50 1.5. The faster one is notorious for optic problems. Both costs significantly less than Leitz 50/2, 50 1.4. Even same optics "quality" Leitz 50 1.5 costs more.
I agree with much of what you say, but I have a slightly different spin on some of the details. There is no doubt the Contax system is lower priced than the vintage Leica system in an overall sense. However,
when in good condition, Contax camera and lens prices can approach that of the Leica system of the time. The quality of both systems is to me inspiring, and although Leica had certain advantages of course such as finders, some would say Zeiss optics were superior (i am not sure myself, but I believe in some instances such a case could be made).
I think there are two main reasons for the value difference that are unrelated to quality. One is that Contax production was quite high as far as camera bodies and their more common lenses (such as the 50mm f2 Sonnar). Another, and I think more important, is that Zeiss did not maintain historical continuity into the modern era as did Leica with their rangefinder system. There were of course geopolitical reasons for this, market reasons, and certainly other economic factors, but this eventually became a significant advantage to Leitz. It was an advantage that was probably unforeseen at the time.
As regards to the 50mm F1.5 Sonnar, I'm not sure what "notorious" optical problems you are referring to. It is an amazing lens performance-wise, and many are still in around in good condition. It is a gem of optical engineering on a par in all respects with the Summicron
of its time. In fact, in my experience, they have held up over the years better than the early Summicron's which are commonly hazed and scratched up to the point of inoperability. In good condition, the 50mm f1.5 Opton is also hardly inexpensive these days - they run around $300 now - cheaper than a Cron yes, but still expensive (at least to me!). The Zeiss Opton Biogon 3.5cm F2.8 is a remarkable lens as well, and really a quite a bargain for its price these days ($300-$450 or so). Other more desirable, but less common post-war Contax lenses such as the 21/4 Biogon, 35/3.5 Planar and 85/2 Sonnar command very high prices in good condition and that undoubtedly reflects user appreciation for quality and performance.