Well, I'd say nothing AND it's priceless.
But as to how far is too far?
I'll use someone in the hospital as an example... Say they were in a massive car accident and are hanging on by a thread. If they have a REAL fighting chance to live and WANT to live, go ahead and give it to them. But if saving them is going to cost so much that it will destroy their family forever and *they* don't even want to live, hell no one else does either because they were a pr*ck, take 'em off life support. It all depends on the specific situation.
But also, I honestly think a big katamari ball rolling up half the world's population and throwing it into the sun to help out with over population would be great. People suck anyway. The Nice Guy doesn't weigh more than the Bad Guy, but it sure takes a lot of him to make up for the jerk. -_-
ALSO, I assume you meant 'pay' for a soul.
WHY, wanna buy mine? MAKE AN OFFER~