Try a Jupiter-8 with MIDI, those sell in excess of 25k. The reason why? There were only 2000 ever made. Additionally NONE had MIDI, you had to get MIDI professionally installed.
AFAIK there are only about 500 in working condition and only about 250 of those have MIDI.
THAT is a synth I’d drop that kinda cash on though…
It’s a JP-8, Moog Modular, Minimoog, Prophet VS, and CS-80, plus all of it’s own original stuffs. Looks like an absolutely wiiicked analogue modeling synth. It’s a tossup between this and updating my Virus to a TI2.
Handmade modular synths can get as big as you can afford (or house). Just keep adding modules and patch leads! Some monsrer modules available, like analog VCOs that can sweep reliable from 0.01hz to 20KHz with no problem. Amazing.