Except a lot of prices increase as the square of the value, rather than linearly -- e.g, a +2 sword is 2*2=4 as expensive as a +1 sword. If fast healing should follow a similar progression, that would make a ring of fast healing 8 16 times as expensive as a ring of fast healing 2.
Personally, I'm not sure a formula will give an accurate value for something like always-on, never-ending (except with death) healing. It is kind of a game-changing item; you suddenly aren't dependent on spellcasters or substitutes for out-of-combat healing. You will be at max hp for most every fight. It's really useful.
IMHO, the limb-regenerating part of the PF ring of regeneration is the least useful of its abilities, given how infrequent maiming is in 3e/PF. I suspect the 90k price tag is just because that's what the ring of regeneration cost in 3.5e.
OTOH, once you have priced Fast Healing 1 at 90k, is fast healing 2 really worth twice as much? It's not much more useful for in combat healing, and until you have hundreds of hp, the difference between the five minutes of healing and ten minutes isn't that signifcant, IMO. And the same logic holds as you go up; sure, eventually, fast healing 10 is awesome in a fight -- but by the time you are powerful enough for that to be sane or feasible, the bad guys are probably doing 10 hp when they sneeze as a free action. So it still is essentially "you get all your hp back after the fight".
So, yeah: picking a price is hard.