I would suggest a lump sum payment option like with the character slots... or perhaps having a special booster for an account+heroes. Such as you have like the 8100 ogrine booster for the 3 month option. You could bump that to a 9k booster for the booster + heroes? Something like that anyway.
Of course we won't be able to see anything about pricing until you release it, but I would much rather pay a lump sum one time than pay paltry amounts monthly.
ESPECIALLY since your Payment Methods are really screwed up right now, and it's very difficult sometimes to jump all of the hoops we need to jump through in order to just buy ogrines from you to begin with!