If DF could combo, I'd place it at 200 PDs.
Because DF cannot combo on Ephinea (which I completely agree with), and because DF's usefulness is greatly overstated by the few who want to feel good about their purchase, I'd say an 80h sword is worth 70 PDs, tops.
I personally hate most DF users because they force everyone else to modify their play style by running around without heals so that the DF user can have that once-or-twice-per-run moment where they one-shot something with its special, only to be killed several times because they're always running around with sub-10% HP.
Sorry; I got a bit preachy there. If you can get more than 70 PDs for it, more power to you. I personally wouldn't pay a cent over 60.