[Taqwus]

Empowerment cost is 50 gems/slaves to get level 1, 30 + (15 * (current_level-1)) thereafter.
Hence, to empower from level 8 to level 9 costs 30 + 7*15 = 135, then it's 150 for level 10, and 165 for level 11.

With Death magic, you should note that boosters exist for every slot except feet.
The most powerful death weapon is the 40-gem 1-hd unique Sceptre for +3 death; unique and non-unique 2-hd staves provide 1. A Skull Cap boosts by 1 for 20 gems. A Robe of the Archmage boosts all paths incl death by 1 for something like 80 blood slaves and a horror mark. Misc items that boost include rings of sorcery and wizardry (+1 each), and the Big Black Book of Secrets (Death and Blood Magic for Dummies, +1 each).