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CONDITIONS
Current Player kills and razings score is at least 500.
ACTIONS
Modify score for Current Player: Subtract 500 kills and razings.
Modify resources for Current Player: Add 50 ore.
Display for Current Player: +50 minerals!
Preserve trigger.
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Is there any reason why only 250 score is subtracted upon killing 500 points worth of units? And if I set it to 600, it would only subtract 300, etc.
None.
I'm just guessing, but it subtracts both kills and razings total. So 250 of each. Just double the subtraction. Again i'm just guessing.
None.

We can't explain the universe, just describe it; and we don't know whether our theories are true, we just know they're not wrong. >Harald Lesch
Kills and Razings Score does not exist as a seperate score. SC just adds the Kills Score and the Razings Score if you're working with it. So if you subtract Kills and Razings it will subtract half of the value from each scores, as Morphling correctly guessed.
When your Razings Score was already 0 then it won't go negative, so only the subtraction of 250 Kills Score will be in effect.
What you have to do is to convert Razings Score into Kills Score*, and only subtract Kills Score.
*Make a seperate trigger(s) like that:
If Current Player has at least 100 Razings Score,
Subtract 100 Razings Score for C.P.
Add 100 Kills Score for C.P.