Yes, in the late 1970s and 80s there was some strong weed coming out of Alaska's Matanuska valley. Yes, people have things they believe are MTF, and yes people sell things they claim are MTF.
But I don't think its clear that there ever really was a unique strain by that name, or that if there was such a line that it still exists in any kind of "pure" form.
Of course some people do believe that MTF is its own inbred line, a hybrid acclimated to growing in AK, but if that were true, I don't think this would be so scarce or "mythical". Some people think that MTF was just the name given to the best commercially available stuff from that part of Alaska at that time; in other words, it was just something else (or more than one thing at different times) given that name. Some think this was a clone-only cut that's long since gone.
I don't really know the truth here. . .and I don't think anyone else does either. We're talking about weed mythology from Alaska from 30+ years ago.
There is a bunch of recently grown stuff
called MTF that was tested out at SC labs ranging from about 15-27% THC, with most of it at around 20-21%. Although 27% THC is pretty darn high (see here:
http://www.scanalytical.com/sample-details.html?task=sample&sample=130730V052 ), it seems that at least in terms of commercially grown stuff, the majority of buds going by the name MTF aren't any more potent than any other top shelf weed.
So all that said, personally, I'd be pretty reluctant to shell out astronomical amounts of money chasing what amounts to a ghost story. Even stipulating that what Dr. Greenthumb sells is excellent, and that its real (which is legitimately questionable, I think), there are a *LOT* of great genetics out there nowadays, most of which cost way less than his asking price.