Games : When is a Reaper considered cost efficient?

The Reaper in early game it is usually used for scouting and harassing the opponent.

I understand that a scan costs 270 minerals and a Reaper can save this investment, but you could also play "blind", so I do not want these 270 minerals considered in the answer.

As I see it there are two scenarios: A the Reaper kills stuff and survives and B the Reaper kills stuff and is killed at the end.

When is a Reaper cost efficient in both scenarios A and B?

More specifically I want to know how many enemy workers I have to kill to break even.