Interesting mechanic.
Revised Death Rules: Fighting For Your Life
If a PC suffers an amount of damage from a single attack (let's say, a Fireball spell or something similar) which reduces him from a reasonable amount of hit points (50% or more than the PC's total) to negative hit points below -10, the player is allowed one last chance to save the character's life.
For each negative hit point below -10, the PC must expend 250 XP. (if he fell to -15, for example, he would need to expend 1,250 XP). After that, he is allowed a Fortitude save (DC 15 plus the ammount of damage below -10). If the save is successful, the character recovers to -9 hp, and the normal rules for recovery and dying do apply. If he fails, the character dies. Why: Sometimes the damage caused by a single attack (like some monsters' natural attacks and some spells, like fireball) may kill even a high-level character. While this certainly enhances the thrill, is anti-heroic. Heroes should be allowed to die heroic deaths, and this kind of situation does not allows that.Pro: This rule allows the player to make a last attempt to save his character, but at a terrible cost. It give players a sense of control over their characters' fates, and helps to enhance the heroic feeling of the game.Con: This rule, if misused, may convey the players the sense their characters are invincible and thus, ruin the whole point of the game, that is overcoming obstacles and facing enormous risks.
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I think I will add this to my own house rules once I have thoroughly ascertained how it will impact the campaign. It seems to me that your concern about it letting characters cheat death too easily is not such a big problem, after all, if they fail the save they still die, in spite of being prepared to spend all that experience. Also, there are enough magics in the standard rules that permit characters to be brought back from death as it is, I don't see that this unbalances the situation terribly. Heros should indeed do heroic things like not quite die in the face of terribly overwhelming odds; this rule gives players a chance to make that happen when it otherwise might not.