When I sold Niflheim T9 about 6-7 months ago the usual bids varied around tt+25-40k. Niflheim has superior dps comparing to Sulfury, but also requires higher longblades level.
Based on that and also that it seems like UL swords seems a little bit more in demand now I would guess that a high tier Sulfury should go for about tt+20-25k and t0 about maybe 10-15k.
I guess highend longblades and shortblades are valued low because no ammo burn. In my opionion these high end longblades should be valued high for that they are one of the few options to skill towards mod nano. There are of course L blades but are you willing to pay insane MU to skill towards level 150?