So, I have been thinking that continuing to double the xp cost for creatures...just because...seems really harsh, and in some cases, just weird.
So, I was thinking of making xp flat after 9th level, just like everyone else, but maybe charging 250,000 xp.
Is this going to break anything?
Depends how you define ‘break anything’.
The game will remain playable, certainly, because these transformations are rare and high-level members of them are even more rare.
If you have a lich who’s been hanging around for ten thousand years, the current rules explain why he’s only 20 HD; he needs ten billion more XP to level. Changing it to a flat value loses that coherence. If you don’t have any such characters, or don’t mind that loss, then you’re not losing anything important.
That said I’d probably go higher than 250k; isn’t that what mages pay? If you make it the same as mages, you remove any incentive for a mage to hold off transforming, they’ll just want to transform as quickly as possible. With the doubling, a mage might want to reach a higher level of spellcasting ability first (especially with how many spells you gain going from 11 to 14), but if it costs the same XP there’s no drawback.
Mages pay 150,000 so this is still a pretty signifigant step up!
Another explanation for said Lich could simply be that he doesn't adventure much and even when he does, it's not like there are tons of 200K+ treasure hoards...at least not by default monster treasure allocation!
Anyway, thanks for your input!
While it’s true that 200k+ treasure hoards are rare, remember that GP from adventuring isn’t subject to the GP threshold. He might just go kill some kobolds once a month for a few hundred years out of boredom. (Not that this is a big deal either way. And you might have already known that.)
Mages at 150k means I’m fine with the idea of 250k though, misremembering! 100k more XP/level is a big deal.