I don't play a ton of SC2, but I watch a decent amount of high level matches on Youtube so I've picked up some general game knowledge over the years.

Obviously inflicting eco damage is important but I'm having a hard time wrapping my head around some things.

What is considered a good trade in terms of army cost for worker cost? If 500 minerals worth of zealots kill 300 minerals of workers is that worth? Obviously it takes time to replace those workers and you lose mining time that way, but when does it become a bad trade? I'm guessing it really varies depending on the time of the game, how many bases the person has to replace workers with etc, but I just wanted to know if there was any conventional wisdom I was missing.