The final staff adds about 65% damage in a freshly reset game. So the break even point is around 127 undecillion souls. However, upon getting the achievement for 1500 Behemoths (+300% Golem power generation) the staff adds 120% more damage (break even point around 91.3 undecillion souls.) I upgraded my staff a few hours into a game with 100.5 undecillion souls, decreasing my souls to 50.5 undecillion while increasing my damage from 778 quin/sec to 860 quin/sec.
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