The problem with using "Player kills exactly/at least/at most (number) (units)" is no matter what, if you set the number of kills to 1, and you have that trigger preserved, as soon as you kill one unit, it's going to keep giving you minerals.
You should use the "Kills and Razings" score system.
It's basically a preset method of keeping track of kills a player gets in the form of points. The point system, however, is preset by Blizzard, so you have to deal with the 100 points you get for killing a Marine, and 50 for a ling. That's not minerals though. From there, if you want minerals to be involved, you'll have to make locations where the player can bring either their attacking unit, or another non-combatant unit somewhere else on the map to a beacon to heal them, and give them minerals.
You'd want locations called 'Player 1 Minerals', 'Player 1 Heal', and 'Player 1 Reset'.
Condition:
-Current player brings at least 1 Terran Civilian to Player 1 Minerals'
-Current player's Kills and Razings score is at least 50
Action:
-Modify Kills and Razings Score Subtract (Number of points) for current player
-Modify Resources: Add 50 minerals
-Move 1 Terran Civilian at location Player 1 Minerals to Player 1 Reset for current player
-Preserve Trigger
This way they'll be able to control whether they want to heal or get money. That Moving unit trigger could get annoying if you wanted to get a lot of money however, so you could get rid of that alltogether and add something like...
Wait for 4500 milliseconds
Before the Preserve Trigger to allow the player ample time to get off of the beacon. Additionally, you'll have to do something like this for every player, and if you want a healing beacon... well, you get the point.
It's not impossible... just time consuming
