[Cosmos]

^ Lol at economics. And I don't think he actually considered opportunity cost.

Opportunity cost is defined as the next best alternative being forgone, which in this case could be anything considering it is hard to define what the best alternative is.

Ok I realize where your problem lies.

This is what you wrote:

Overlord sac = -8
Replaced overlord = +8
Supply change = 0.

This is wrong. It should be

Overlord sac = +0 supply, (say you're at 10, you made the sac overlord = 18 and after you sac the overlord supply becomes 10).
There is no change/negative in supply after your sac overlord dies because your sac overlord is considered an extra supply.

Replaced overlords = +8
Total supply change = +8.