So frequently, I find myself raiding the enemy base with some fast harasser unit (example: Mutalisks) and I find that the enemy mineral line is defended in some way (example: marines/photon cannons). Now I know that each Mutalisk costs me 100 crystal and 100 gas, but I also know that each Collector costs my opponent 50 crystal + all the resources it will ever collect for the rest of the game. So the question is, how can you determine how many Collectors you should kill for some value of raiding force for the loss of the raiding force to be 'worth it'?