If you're stupid, there are amazing bargains to be had on Chinese Ebay. Item #320009394191 purports to be a 'RARE!!' Buchla 100 Modular Synth. The auction ended recently, perhaps because the 'Buy it Now' price was 8,000 Chinese Yuan, or £542. Yes, the seller has zero feedback (and the last picture is the back end of some home A/V amp). If your tastes are more modern, how about #320009813151 - a Clavia Nord Stage with a 'buy it now' of £94.
Now, in case anyone has missed the subtext of this post: If you try to buy crazy cheap classic synths from Chinese eBay sellers with zero feedback, you will most likely be ripped off. (Thanks, Angstrom)