How much is regeneration spell worth in gp?
Assuming you're asking what it's worth to have a NPC cast it, that will depend on your setting and circumstances.
How prevalent are level 13+ spellcasters in the world? If every city has a few dozen, then it likely costs somewhere between a few hundred and a few thousand gp. On the other hand, if you're dealing with one of the only casters in the world who can cast it, then they can basically name their price in valuables, favors, or both.
Of course, that doesn't take into account circumstances. A high priest of the same faith would likely cast it, on a warrior who lost a limb in service to that faith, for free. While it is a valuable resource, it isn't particularly time consuming, nor does it have any expensive components. A cleric who is in desperate need of funds might cast it for far less than it is actually worth. If a black guard asks a good cleric to cast it for him, it's plausible that the cleric would refuse to do so for any sum. A neutral cleric might find him so distasteful that he requests an absurd amount on the assumption that the anti-paladin will simply leave.
If you need a formula, you could start with something like the square of the spell level times 20 (plus the cost of any consumable components) plus 30. Regenerate would be 1,010 gp using that formula.