If you don't mind on which units you kill, so long as they are counted for whatever you want (money, custom score, exp, ect), then just use the kill score (or Kill and Raze score if you want to include building kills).
If you do it by kill score, after a little getting used to, you'll get used to which unit is worth what.
Zerglins, for example, is 50.
So with your trigger could do this
Cond.
Current Player's Kill score is at least 50
Act.
Subtract 50 score from Current Player's Kill score.
Add 1 Mineral to current player.
That'll give you 1 mineral per 50 kill score. Though you do have to realize that the smallest unit score is larvae (10) and broodling (25), to count those in there or not is up to you, to count those in there, you can make 2 triggers, one setting the At Least option at 10, the other at 25, then remove as much as it says in the condition.
So
Cond.
Current Player's Kill score is at least 10
Act.
Subtract 10 score from Current Player's Kill score.
Add 1 Mineral to current player.
Cond.
Current Player's Kill score is at least 25
Act.
Subtract 25 score from Current Player's Kill score.
Add 1 Mineral to current player.
Now, of course that when you kill something like a ghost (350), you don't want to wait for the trigger to fire itself 7 times, right? So you can just set some higher ones, so you could do someting like:
10, 25, 50, 100, 200, 400, 800
then you just have to set how what's worth what. If you want 50score/mineral, then for the 100, you would have to set 2 minerals because you're removing 100 score points, then if for 200, you would add 4 minerals, and so forth.
Since the triggers run from top to bottom, I would suggest you put the biggest conversion on top then go down, so that it'd take the biggest chunk of score and convert it first and leave whatever is left for the smaller numbers, then smaller and on.
use thermo's method of most kills to cash triggers. if your map has 6 players or less. (read tutorials)
and instead of adding 1 ore change it to add 1 custom.
for what i know its harder to understand reading the tutorials than actually looking at the map because the map is simplfied, so look for the mostkillsmethod map. it should be under related files, on Kills to Cash perfect in tutorials
Thermo's way is the only way to do this really, unless you don't care if the scores you get from units are customizable. The kill score can't make it customizable, since you can't change the kill score each unit gives (without extended actions anyway, but they were patched so don't bother asking about them).