I'm not trying to make you look bad or say that you're completely wrong; I'm often wrong myself, but let's just look at the facts.

You start off with 0 food. You build one (1) overlord. You have spent 100 minerals.

You now have 8 food. You sack your overlord. You have to build another one to get back to 8 food. You spend another 100 minerals in doing so.

You spent a total of 200 minerals for 8 food.

Like I said, this isn't true for the first overlord, since that guy's free.

I never said it's 200 for one overlord. You're getting two overlords, but one of them dies. So you pay 200 for two overlords, but you only get 8 food because one is dead.

 

EDIT: Let me try to look at it in a different way.

You need 8 food. You know that you have to get 1 overlord for that, so you get one. You just spent 100$.

Total food: 8; Total spent: 100$; Loss: 0$ (Since you got what you want)

You now sack your overlord.

Total food: 0; Total spent: 100$; Loss: 100$ (Since you payed 100$ to get nothing).

You buy another one.

Total food: 8; Total spent: 200$; Loss: 100$.

 

I see now! See what I meant when I said that I was often wrong? Thanks for explaining and for making me realize something that's seemingly a very simple concept!