I don't think that most vintage synths are over valued. Some few of them get stupid prices, such as 303's. These are the exception, driven by fads and hype. Classic synths are a rare commodity, there will never be any more of them appearing, their numbers are finite, yet the number of buyers is always increasing.
Yamaha made very few CS-80's, Moog made very few Multimoogs, Roland System 700's were made in the hundreds. There machines are old, rare, and especially, they sound fantastic and are still totally useable, and sound (to most people) far better than todays brittle digital DSP boxes and virtual instruments.
Look at an MS-20 or a Roland SH-2. If you look after them, they are incredibly reliable, needing only a re-cap every 20 years which is no big deal. There are no electronic parts in them that you couldn't easily find or sub, even the Korg 35 filter is simple to build from scratch. They sound fantastic, are engaging to play, and are a joy to use. For something like that, $1000.00 to $1500.00 is not a lot of money, considering what you pay for a modern plastic digital machine that is worth nothing after a few years.
Don't forget also, not all vintage synths are expensive. The good ones fetch good money because they are useful instruments that are sought after because they sound great. Think JP-8, Minimoog, MS-20, Prophet 5, ARP Odyssey/2600/2500, SH-2/5/7 Octave Cat, PPG, Korg PS series and so on. On the other hand, think of all the machines from that era that no-one pays much for because they were dogs, didn't sound good or were just c**p - Poly 800, most stuff from Casio, Technics, all the "badge engineered" machines that ARP and sequential had made in Italy by Seil, and machines which should have been good but just sounded a bit lame like the Yamaha CS-20M. This stuff doesn't fetch much at all except from people who don't do research and think that anything old must be good.
If you think old synths are expensive, go spend some time in old guitar world.... Yes, you will find some desperation sales going on in these tough times, but as most sales seem to be auctions now, the really good machines will still go for a lot because those with money will still compete for that which is rare.
Synthesizer service tech since 1982.
Synth parts and service, Sydney Australia.