How much should I pay for a 2001 Thiel CS 3.6?

Buy them, they are classics, I had those in my system for 10+ years. Be patient, upgrade upstream slowly and they will give you years of enjoyment.

http://www.stereophile.com/content/thiel-cs36-loudspeaker-measurements

I used the 3.6 for about 8 years, and it is a very good speaker in many ways. It is quite demanding in amplifiers though. If you go with a solid-state amp, you will find that you need lots of power. I started with a Classe DR-10 with about 200 watts at 4 ohms. It tended to sound too restrained, like it was holding back, so I traded up to a Classe CA-300 with 600 watts at 4 ohms. It sounded much more powerful but even with those reserves the 3.6 still sounded like it was holding back dynamically.

I ended up trying a number of tube amps ranging from Cary CAD-100 monos to push-pull 300B monos, but in many ways the best sound I ever got out of the Thiels was with a bone-stock Dyna ST-70. Sure it wouldn't play as loud as the big Classe but the overall tonality and dynamics within its power range was superior.

So the Thiels are very good but don't be surprised if you start getting the urge to change other equipment to make them sound fuller and more dynamic.

FWIW, many B&W's aren't the easiest load for amplifiers either.

I will pay double the full price for a NOS 3.6! They are that good but note that Thiels do not have any stock tweeters left for replacement! You may want to check this out.