Tesla Tubes

Hard to generalize. Gold pins are presumed better due to their corrosion resistance. Diamond stamp is AFAIK, refers to the mark molded into the bottom of Telefunken tubes. Not sure how it applies to Tesla tubes. Also be aware that many of the older tube brands have been sold, with the buyer getting the ability to use the name, but are using factories in other places, primarily Russia.

There is no guarantee that NOS= Old but never used, or just Old stock tubes will sound better.
The gold pins are as Swampwalker says and I might add supposedly better tubes all around. Also the diamond between the pins on the bottom indicate made by Telefunken.
I became a big fan of rolling tubes (changing them to another brand even type) after hearing how much better my first tube amp sounded with tubes manufactured in the 1940s, 50s and 60s.
So if your willing to hear sound with no or only moderate improvement for you money you might want to try it.
Be sure that you are buying one of 2 possibilities . The first that you are getting tubes from Tesla made before 1980or
Currently manufactured tubes made by a company sometimes called JJ Tesla. They are JJs which are good tubes but you can buy them for very reasonable prices, if you search the "interweb". I find an outfit called Eurotubes does a very good job with JJs and ships very quickly. I have no association with them at all financial or otherwise.
BTW which tube type are you hoping to buy?

At the time I was thinking about buying a ARC LS2 pre-amp and replacing the tube (tube ECC 83). I ended up changing my plans. However I replaced the Tube in my DAC, (12AX 7)with a Mullard and was not that impressed. Thought about trying a Telefunkin model. That's were I got confused over the Gold Pins, the Diamond and the age and region of were the tube came from. Is there a certain country that is noted for there tubes? Russia, Germany? Thanks for all the help

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.