Heh, if your only builder is a griefer, you've got bigger problems than no team cash.
The net effect of this idea is to level out the human team so that better players are worse off and worse players are better off, as well as making things easier for late joiners.
There are a few other ways you could do this effectively:
The simplest way is to increase the price of all weapons at the start of each match, and gradually decrease the prices automatically as more people buy that item.
A better option is to research certain items at the armory with a seperate menu: A good player who is fully kitted out and still has 2000 cash could decide to spend some of that cash to make certain items cheaper. A team message would appear:
"Pvt. Hudson has researched cheaper shotguns"
"Research" implies that shotguns would be a little cheaper for the rest of the game, though and I think both the above methods would cause balance issues.
A "Subsidy" alternative would be the best option, I think; a player can put c500 towards subsequent purchases available to anyone. Spending c500 on a shotgun subsidy would make the next 10 shotguns c50 cheaper.
Subsidies can overlap, eg. since shotguns only cost c150 by default, spending c500 on shotguns 3 times in a row would put 10 free shotguns in the armory. This would allow players with excess cash to influence the choice of items available to latecomers and poor players.
I know that given a bit of spare cash in game, I'd always stock the armory with free helmets.
