Generally speaking life per kill doesn't add much value if any.
Items with high ee like PlayTheGame was suggesting already have value, but whether ee is considered high depends on the level. Generally speaking, even max ee low lvl (tier III, tier IV) items don't have a ton of value, but if they have good side mods on top they could have a little more. Life per kill is a pretty good side mod on tier III, and IV items if they are already good without the life per kill, so you might see some increase in value there.
Starting from around tier V, or lvl 10, items, 100+ ee is possible, and you'll see those items start to have good value, assuming they are good types. If they happen to have a high life per kill property on top, their value might be a little higher...
You can refer to this chart to see the max values of properties at each tier (magic luck allows you to rarely go 1 tier higher, or overspawn):
