Tesla Tubes

If you want to try nice 12ax7 ( ECC 83) tubes you should give the
Sylvania Baldwin green label or the GE from the 50's or 60's a
try. I think they are better sounding than any of the newer tubes
produced now. BTW,these are made in USA and the prices on
these are pretty reasonable.

Rsa, Mullard tubes and Telefunken are at opposite ends of the spectrum. Mullard are warm, smooth, with a classic tube sound. Telefunken are more detailed and open.
Both are expensive premium tubes and are a risk to purchase without knowing their sonic signature.

Brent Jessee provides a good reference for tube types, and how the different brands sound...

http://www.audiotubes.com/12ax7.htm

I'm a big fan of rolling tubes, but don't pay top dollar for NOS tubes without a recommendation.

What would be considered a Fair price for a single tube?
Thanks for all the input!

Be aware that some circuits really respond to tube rolling while others don't. My own experience with ARC left me thinking they are not for the tube roller (input and small signal tubes).
A fair price is difficult to know, several factors go into the asking price, generally how well the tube tests is a big factor but prices are a moving target. In general prices for these tubes are climbing as they are getting used up, and I think people are more aware of tube audio.. The ECC83 or 12AX7 is a popular tube. The Mullard and Telefunken will command a higher price depending on condition and variant, than the GE. You really have to check around. Visit sites like Brent Jesse to get oriented. It doesn't hurt to watch the tubes for sale on this site for a bit, to get a sense of what the used tube market is like.
Be aware that many tube sellers are not all that well informed themselves and terms like NOS are used loosely by some. Buying old tubes is not like buying gear from a manufacturer, but some well established sellers will warrant what they sell.