Probably because it's a 100k Mob Boost Skill. The avg price for a 10k use II skill is anywhere from 200-500m this one is probably on the lower side so for a 10k use of this I would say it's worth 250m. If you do the math it's 2.5b but no one would pay that much for it. Not that anyone has the need to want to have this skill but nonetheless it's still worth quite a bit.