For example, if the government spends $600 on a hammer, and the hammer-maker gets his $20 for it, what happens to that other $580?
If the contractor bills $600 for a government hammer, he gets $600 for a government hammer. Why do you think he's only getting $20?
I guess it could've been phrased better as it
costing the government $600 to give the contractor $20 for a hammer. I'm under the impression that there is a cost to running the government, itself, and that it can be terribly inefficient in how much it costs it to spend money to buy things. I don't know how bad it is, and I doubt it really costs $600 for a hammer, but I don't think that all of the "spending" (or costs) of the government contributes to private income.