One of the ancients in Clicker Heroes is named Atman and he costs a large number of souls to level up. His impact on your game is he makes it more likely to have primal bosses spawn as you're playing. Primal bosses are one of the two ways to earn more souls so he felt like a pretty good guy to have. Spend some souls now, get more souls later! The problem is not spending those souls makes you do more damage, which means you get to kill more things, which might well generate more souls themselves. Almost certainly I shouldn't have bought him when I did, but that ship has sailed. Now I want to at least figure out how many souls I expect to gain from an additional level in this guy so I can decide if I want to sink more souls into him now or not.
Typically on a soul farming run I'll get to level 145 and then reset. This gains me 8 souls from other sources and then some number of souls from primals I find from 105 to 145. My gut feeling is that's been about 4 extra per run. How does that stand up? Well, by default each boss has a 25% chance to spawn as a primal and is worth some number of souls based on the level. In this level range I have 4 shots at 1 soul, 3 at 2 souls, and 2 at 3 souls. Which actually averages out to 4 per run. Anyway, that also means that adding an extra 1% chance of spawning a primal on each level will gain me a whopping .16 souls per run. I can do a run every half hour, so if I were to spend the 23 souls on the next level I'd expect to make them back after 144 runs. So about three days to break even, assuming I didn't lose any time by giving up 10% of my damage, anyway... Which all seems pretty terrible, actually.
But what if I go deeper? My longest run ever killed the boss of 260 who is actually worth 13 souls. The total of all those souls could total 194. Getting an extra 1% of those would get me almost 2 souls per run, but that run took me a day. That's 11 and a half days to break even, which is worse than the fast run case.
I'm all about long term rewards, but a week and a half to break even on the cost even if it didn't drastically reduce my damage is a bit much. Until I really ramp up my ability to farm for souls I don't think I should be sinking any more souls into leveling Atman up any further.