I imagine it'd be like a blank cheque. They just keep buying with no real intention of paying anything of in the short term. After the war many companies would demand payment and get very rich from the war.Unless orbital elevators and surface-space cargo shipping reduced costs THAT much, that's insane to think it was only that expensive. We need to see some economics in upcoming Halo books. I'm interested in seeing the cost of each ship, Infinity, and how much debt the UNSC racked up. Would that even be debt if you're fighting for the survival of your own race?
With an economy spanning of 800 planets (not including inevitable mining operation and outposts) the UEG, and so UNSC, would have a gigantic budget.