ring of regen regrows limbs, and is way overpriced for what it does anyway (since nothing really cuts off limbs in a useful fashion that ability is moot, and gaining hp every hour as though you rested a night is weak when you can afford a 90k item.. at that point if you are relying on hours and hours to heal you then you are in trouble.. especially since you could've gotten a real healing item for less
)
But yeah, I think it would wind up being more than the price for ring of regen anyway.
Maybe something like 100k + 2*(fast healing rate)^2
So this item would be 150k.
Going by spells and such I believe there is a 1st level spell that gives fast healing 1 for about a minute (dont have the book and going off of memory so this may be completely wrong). That would put the price for the item 'very' low.
Looking at the uses of such an item though, fast healing 1 means almost nothing in battle.. basically you wont bleed to death and for really long battles it might net you a few hp. Also, so long as you have a few minutes to rest up before your next battle you dont need to be healed (10 points/minute = 600 points/hour, raise your hand if your character wont be completely healed in an hour so we know who is 'way' into the epic range). So this item would basically make it unnecissary for you to recieve healing outside of battle most of the time. The price of 102k given above seems all right for that
Fast healing 5 is actually more useful in battle, as it is a psuedo DR 5 against any one attack. So, the higher the plus gets the more useful it is by far. Also, you have to wait much less time to be healed (3,000hp/hour? if it takes more than an hour to get your hp back now I dont think you are playing d&d anymore..lol)
So really, just off the top of my head I'd go with the 100k + 2*(fast healing)^2. There is likely a better way, or more useful calculation, but there is that at least.