Or just give both options. Have one price for per-character, and then, say, three of four times that cost for legacy-wide. That'll make both the single-char players happy and the alt-o-holics who make more than 4 characters happy.
Yeah, I'd be fine with that. I actually think they should do that for a few of the character perks. Instead of making you buy them for eight separate characters, maybe offer them as a legacy-wide perk, as well, for a cost equivalent to, say, the per character cost X 5.