After buying, selling and watching synths on Ebay for months and months, i can state with certainty, what the most expensive, vintage, used, non-modular analog synth you can buy (if you are lucky to find one and the money to purchase it) is.

A Restored Moog Minimoog Model D with new exotic wood case?
nah..

An ARP 2600P v3 with 3620 keyboard?
nope..

How about a synth that was mostly used to create cheesy UFO and LFO synth effects and is almost unusable to play music due to its unstable oscillators and tuning methods?
Pound for pound, the most expensive vintage-used-non-modular-analog synth in the planet, going for an astronomical $7400 on Ebay is the EMS VCS3 mkII synthi cricklewood w/ keyboard & pins.

The VCS3’s has made appearances in albums such as JM Jarre’s Oxygene, Pink Floyd’s “On the Run” from Dark Side of the Moon and many others.
Obviously this synth has made the transition from being a useable musical instrument to a museum piece of synth history, which is probably the only explanation as to why would anyone on earth pay so much for it, but i am pretty sure hype as lot to do with it.
For the money, I would take an ARP 2600 any day over the EMS VCS3.. or two Minimoogs and have a couple of grand left to buy other vintage stuff..

2010-02-01 UPDATE:
An ARP 2600 (Black/Orange) w/ 3620 Keyboard auction has tied the record of the EMS VCS3, going for $7411 in this auction