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.
Best Answer
It depends on a lot of stuff. What your plan is, how well you use it, what the enemy is doing.
You also need to consider that part of a reapers resource cost is time. A reaper takes up time which you could have been using in order to build marines or something else which could be more important.
If your plan is to do a strong one base attack then it is highly unlikely that a reaper's scouting information will be worth it. Because regardless of what you scout you still will be going for this attack. If you happen to distract the enemy and maybe pick off a few workers especially scvs that are building something then you are pushing back your enemy and improve your attack window.
You don't even need to kill a single unit in order for your reaper to be considered "worth it". For instance with your reaper you could gather information which is worth far more than the cost of the reaper. Maybe your reaper spots a dark shrine which prompts you to get detection thus preventing you from losing the game as well as making all the investment that your enemy has put into the dt tech essentially worthless. Or maybe you spot that they're playing passive or aggressive and it allows you to react accordingly.
Reapers are also good for gaining map control and especially towers from zerglings. Meaning you have more warning if the enemy decides to attack.
How much value can you put on information? A reaper is a very strong tool in determining what your enemy is doing which allows you to react in better ways than you would have been able to do otherwise. If you can get worker kills with it or pick off a stray zergling or zealot here or there, then that further improves the value of the reaper.
So in short you cannot easily answer this question as there is too much to consider. Obviously it's worth more if it kills stuff and survives because you can continue to use it for map vision and scouting and harassment.
Information is almost always going to be valuable but it also depends on how you make use of what you see. Being able to determine a good response to what you see will determine if that reaper's sacrifice (or not) was worth it's cost. An easy example is if your enemy is dt rushing, you see the dt path happening and so you defend it easily, counter push and then easily win because their dt rush cost them so much but didn't yield anything because of your reaper scout.
In regards to reaper vs scan. Again, it depends. If you are planning to do a fast attack then building that reaper means your attacking army is considerably weaker so it would be better to scan. But if you are planning to do an attack a bit later then you can use a reaper to scout and slow them down. As you want the extra money to set up a stronger economy while delaying your army slightly.
I'm sorry my answer isn't very specific but there are so many different scenarios and factors to consider that it makes answering this question impossible.