A DC for such a case should be astronomical. Still, by the time you have PCs able to make such checks with real chances of success, you'll also have casters with Charm Monster, Dominate Person etc etc...
That's true, but there's a few things that you should remember about those spells, or magic in general.
First things first: according to the rules, the Bluff skill doesn't work in that manner. Secondly, Diplomacy and Intimidate affect NPC attitudes, but they do not allow you to dictate NPC actions. The Charm spells do the same thing.
The Dominate spells are powerful, but allow for clauses where the creatures do not carry out obviously harmful effects, and also get a new saving throw at a bonus to negate the spell if ordered to perform an action that goes against its nature.
However, the spells get a saving throw, where the skills do not, which I think was the original point of NewJeffCT when he made this thread. It's true that the Charm and Dominate spells will be very effective against creatures with low Will saves. There's no objection to that from me. However, any creature that maxes out something like Diplomacy can affect all creatures, regardless of level or hit die, just as easily. By RAW, and with only the PHB, I can get +23 to Diplomacy as a half-elf by level 3. No flaws, for extra feats. No splat books. Just the core PHB, +23 to Diplomacy by level 3 (and that's not counting in age bonuses or anything).
This causes a problem. People don't use the term "diplomancer" for no reason. If I can take someone from Hostile (will take risks to hurt me) to Indifferent (doesn't care much) on a natural 2 or better at level 3, no matter how high level the creature is, with no save, that becomes a balancing issue.
No, it's true that I can't dictate his actions. But I've definitely influenced his attitude towards me. Maybe I have to make the check in 1 round, rather than with a full minute. I take a -10 penalty to my Diplomacy. That means that at level 3 my half-elf bard can make that check on any creature he meets on a natural 12 or better. That's 45% of the time. 45% of the time, the level 3 half-elf bard is so good at saying something within 6 second that Asmodeus himself (or Orcus, if you prefer) goes from "will take risks to hurt you" to "doesn't care much." That's massive.
That's why I think it's able to be abused. No, you can't dictate NPC actions. You can, however, reliably make NPCs act much more civilly to you, even by level 3. An unlimited number of times per day. To any creature, regardless of power. With no save.
So, yes, Charm and Dominate spells are powerful, but Diplomacy, as written, isn't balanced with them (in my opinion), it far exceeds them.
But, hey, that's just my two cents. As always, play what you like
