Honestly i believe it is a mix of two things. One is demand, the second is weight. I waited about a year for an early model T-verb head to pop up - wanted one for years but they are getting very hard to find - let alone tremoverb heads in general which is why they are fetching more.
Nothing wrong with the tremoverb combo's and i have seen a few of them that if you know what details to look for, the amplifiers themselves are as good and also in better shape than an equivalent 120W early model T-verb. I just prefer the head cabinet design for a guitar amplifier. Nothing wrong with head-mounting a tremoverb combo either - but people are picky and this is a buyers market.