You should use hyper triggers with any kills-to-cash system to improve its accuracy. If you have leftover scores with the system I posted, you can duplicate triggers to make it better. Add some triggers for each level, as follows:
Conditions:
Current level is <unit x>
Current Player kill score is at least <kill score for unit x> * 4
Actions:
Subtract <kill score for unit x> * 4 kill score for current player
Add 4 custom score for current player
preserve trigger
Conditions:
Current level is <unit x>
Current Player kill score is at least <kill score for unit x> * 2
Actions:
Subtract <kill score for unit x> * 2 kill score for current player
Add 2 custom score for current player
preserve trigger
You should double the multiplier for each trigger, and make sure to start with the biggest one first. Just by adding these two triggers for each level, you will improve the accuracy from 1 kill/trigger cycle to 7 kills/trigger cycle.