i appreciate the help guys

If you're not using every single unit in the game, you could use units each with different kill score, using heroes versions if you need a unit that already has a killscore. Use the killscore method from sacrieur's link.

That's how I did in my Space ship RPG that nobody ever played anyway. It had ammo calculation and counted after a kill. Not the most accurate system, but it still works. It's perfect for a kill for cash system though, given that you don't need too much units.