[HaNdFisH]

Okay

So you say have a choice to either
A) Ovie scout
B) Do an evo chamber scout

Option A)
You make 2 overlords, scout with one lose it, other provides supply
Cost 200 minerals, have your standard 8 supply

Option B)
You make evo in their base, costs 250
You still need an overlord for supply though, so another 100
cost 350 minerals total

I really don't understand where you are coming from. Yes if you make 2 overlords and lose one you are up 8 supply that is true. If you are planning on losing an overlord scouting you typically make an extra one, this is where you pay the "cost" of the overlord and it is 100 minerals. This is similar to paying for the hatch cancel into evo chamber which is 250 minerals.

You can make the new overlord so that it pops before the old one dies preventing any supply block from occurring. If you also require an additional overlord to provide extra supply you can make that as well.