use the program SCtrigger (find it on yahoo, I don't feel like uploading it, lol), it automatically creates the triggers of giving x minerals or gas every whatever kills. in general, this is what it does automatically once you fill in the fields:
Con:
player x kills atleast y units.
Act:
add 1 resource to player 1 or whatever the hell the trigger is
then in the next triggah it does the same thing over again only instead of, oh say 10 units, 20. then u can set it to automatically make these triggers as far as you want it to go. a really nice program, really, because if not you would have to make 1 trigger for every single mineral or gas the players would get, which would take a hella long time. if you cannot find it after a good effort looking for it reply and I will email it to you or upload it so you can download it.
vindexus has the best point, though. a lot of times, expecially in RPGs or something, it goes by kill score. in general the kill score of a unit, if costs no gas, is twice the amount that it costs to build in minerals, broodling 25. and a few others I know off the top of my head are hydra 350 and goon 500. ultra like almost 2000 or something, I don't know. unless it is a defense map and you want 1 mineral per 10 kills or whatever you want, I would just use score.
good luck!