Interesting responses. thanks for the input.
I introduced this item into my campaign as part of a treasure pile. i had marked it on my sheet as a cursed item with an approximate sale value somewhere in the 2500 gp range.
Why?
The penalty is way disproportionate to the gain.
Most of the big bad reflex or half aoe spells go in the beginning of a combat. While some do go off afterward, counter spell tactics and closing with the enemy makes the shift to targetted spells more likely after the beginning. So for my comparison i used 10th level characters starting somewhat fresh.
The normal results from say an empowered fireball is 52 base damage.
This means the typical save results are:
roll 1 take 52 hp (often -5 for EE)
roll failed take 52 hp
roll made take 26 hp
These numbers are fairly straightforward. The 26 hp will sting but will almost never put someone in danger of death. The 52 ho will hurt a lot but is also not likely to put anyone in danger of death unless they are an unbuffed unprotected mage, which rarely occur among my PCs.
With the ring this changes to...
roll 1 take 104 hp.
roll failed take 62
roll made take 26.
Now IMX the difference between 26 hp to start a fight and 0 is definitely an issue but it is not a life threatening thing. it probably means someone loses a round curing you in combat if things get real tough OR you need more cure spells once the fight is over, burning more of a CLW wandf's charges.
However, 104 HP is a very definite lethal threat for most any 10th level character, even from full. If 52 was dangerous, 104 is just plain lethal. Only fighters with high CONs are likely to be able to survive that 1.
Death is not curable with simple cures. Death costs permanent loss in Xp and level. Damage such as the 26 hp can be cured and frankly the number of times a MADE save for a reflex spell resulted in critical failure are very small.
So i saw this item as a cursed item, one that actually offers some benefit to lure in its victims and will help them but which will inevitably result in their death.
Its kind of like betting someone with odds of say 10 to 1 (assumes you make the save on an 11+ roll) that you will either get free stuff on 11+ (what is the price of say 6 CLW charges off a wand to replace the 26 hp?) or you will be killed on a 1.
Expected yield (1/20 x 52 more hp compared to 10/20 x 26 less hp) fails to work out when there is a threshold such as death involved.
hence my conclusion.
Here was how my logic went..