You'd probably do best to convert kill score into minerals.
Have a trigger like:
QUOTE
Current Player kills score is at least 100.
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Preserve trigger.
Modify score for Current Player: Subtract 100 kills.
Modify resources for Current Player: Add 1 ore.
You won't be able to manually set how much each unit is worth, but instead you can scale the unit's power to match their mineral value.
For example, Zerglings give you 50 kill score when you kill them. You'd need to kill two zerglings to get 1 mineral.
Hydralisks are worth 350 kill score, you'd get 3 minerals for killing one, and have 50 points left over.
So, what you can do is make Hydralisks about 7 times stronger than zerglings so that the rewards for killing them are balanced out better.
The upside is that you only need this one trigger.
See
this list to find out how much kill score a particular unit is worth.