You only pay for the last upgrade in the list, SO SAVE UP.
BTW boosters are one of the most powerful upgrades, at max level you can get through almost the entire level with virtually noo effort.
Pretty good way to grind for cash: set map to frostbite: create a 6v6 CTF game and set Ai to very easy. One direct hit from a rocket launcher is a kill, try to get a huge mutikill!
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