Well, if you go by the way sacrifice works in the movies, some of the existing factors are very appropriate. Remember the temple scene from Indiana Jones and the Temple of Doom? You probably want a ritual with several minutes of casting time, and a requirement for several dozen flunkies to assist. Those DC mods are in the book.
As for the sacrifice itself, that's necessarily an ad hoc modifier, which makes it a DM call. (Maybe this thread should be in House Rules?) You probably want to consider the rarity of the required victim when assigning the DC mod. If the requirement can be filled by any random humanoid, maybe make it -3. If it has to be an adult human virgin with Charisma 18+, maybe -15. If you can only use left-handed red-haired half-celestial drow paladins, then -30. (I'm pulling all these numbers out of the air. My point is that restricting the allowed sacrifices makes the spell less useful, and should thus reduce the DC more.)
I'd give an additional bonus, maybe -15, if you specify that the victim must be willing and cannot later be resurrected (even by Wish or Miracle). "Willing" in this case means that they cannot be forced, charmed, dominated, blackmailed, or in any other way compelled to take part.
Note that epic spellcraft DCs are specified during the creation of the spell, and cannot be changed later. Say you create a spell that summons the King of Hell, but requires the sacrifice of a 20th-level victim. You cannot use a 19th-level victim to summon a lesser monster; like all epic spells, it's all or nothing.
