I have paid up to 14 USD per USG (Netherlands, 2009). It can go a lot higher, in further out places like Turkey or Egypt where you could hit 25 USD but to be honest some of those are probably local scams.
Whether the TB GT (post year 2000) is faster than the pre-GT would be hotly disputed by pre-GT owners

I know some have disputed my speed data (low level, 23", 2300, 11.0GPH = 138kt IAS) but the ASI has been checked using the 3-headings / GPS GS method and is accurate. However, I do know that you only need one of the flaps to be hanging 2mm low and the whole thing is flying sideways (true for any airplane I am sure). I also rear-load; if flying 2-up, the back seats are empty and there's a pile of junk all the way in the back.
As to sales, this spotter's URL
http://www20.brinkster.com/dlenton/search/tbshow.asp?start=2000&count=50
(edit the URL as desired to increment the starting number from 2000, to 2050, 2100 etc as desired) suggests that about 230 "GT" airplanes were made (serial numbers 2000 onwards) and most of them seem to be TB20s or TB21s. On a quick scan, a lot of these were either made N-reg and stayed on N, or (like mine) were later transferred to N, so there should be a fair # kicking around the USA. Mind you, nobody can tell (without a lot more digging around faa.gov) how many of the N-reg ones are actually in Europe. Probably 50%. The TB21 in particular never made any sense in Europe unless you had the full IR, which very few pilots have.