In the mid 80s analog synthesisers fell out of favour as the new digital models were perceived to be superior. Listen to the Yamaha DX7, the most popular of the new breed at the time and compare its resale value to some of those older analogue synths.
It may be that the DX series has become a bargain just as a MiniMoog Model D may have been a good buy for $500 in the mid 80s.
Before paying over a $1000 for a Roland TB303 or TR909 research the price for these toys when they were new.
Even some 90s synths are now very cheap on eBay.
Keep in mind that it is the filters that give these old synths their characteristic sound and there is no point paying for a filter you dont like. Often the synth that does not impress you when played solo can sound great in the context of a good track.
For example, compare the old Sequential Circuits Pro-One to the new Dave Smith Instruments Mopho. They sound very similar but the newer model can save sounds and has midi. Now compare the prices.
Finally, these synths are capable of millions of sounds, most of which have not been used, so bear in mind they sound good because someone found a musical use for them.