Actually, as a 1.25% heal, Grasp of the Undying heals 50 per 4000 Health. If ranged (0.625% heal), it's 25 per 4000 Health if you're ranged.
In a period of 60 seconds, assuming you land every single Grasp proc the exact moment it goes off cooldown, that's...
4000 Maximum Health
Melee: 4000 x 1.25% = 50 x 15 = 750 Health healed over 60 seconds
Ranged: 4000 x 0.625% = 25 x 15 = 375 Health healed over 60 seconds
Let's say you're a squishy champion, however. So.. 1500 Health sounds good.
1500 Maximum Health
Melee: 1500 x 1.25% = 18.75 x 15 = 281.25 Health healed over 60 seconds
Ranged: 1500 x 0.625% = 9.375 x 15 = 140.625 Health healed over 60 seconds
Something tells me Grasp of the Undying, in its current state, is not going to be able to be fixed with nerfs alone. Nerf it too much, and it becomes useless by both the intended users and the abuse cases.
Though to be honest, I'm not entirely sure which champions are actually intended to use it at this point. It appears to be a tank/bruiser mastery at a glance, but in practice, it seems like it's useful for most anyone.