[BigDaddy]

Empower costs ~35 blood slaves. 25 slave booster costs ~32 before forging bonus (if any). The increase between B2 and B3 (effective) is about 1.4 blood slaves. 25 turns for empower. ~23 for brazen vessel. Now, boosting further could be done in a cost conscious way with a blood thorn, even to B4 effect or just 1 more B for any B3+ blood hunter. In those case the RoR is equal to the forge cost of the booster. Armor of souls with a 50% forge bonus takes only 25 (adding the cost of the forger's turn) turns to pay off for even your b6 souped up warlock.

Even a modest 25% forge bonus makes the bloodthorn and brazen vessel attractive, especially for low level blood hunters, especially earlier in the game, if you're in a good position.

Also, saving can be achieved if you only forge on turns you've been unlucky and caused an unhealthy rise in unrest.