the worth of the guns and tanks and other things we’ve been giving them that were just collecting dust over here?

Use of reserves motivates replacement. Just because you're giving them weapons that were produced in the past, and therefore whose (production) cost has already been incurred, doesn't mean that occurs in a vacuum. With stock running low, contemporary money goes in to replenishing that stock. In effect, there's no difference whether you send old or new equipment, because both incur costs in the present.

No actual money was involved and so didn’t really cost us anything.

It cost you exactly the amount it cost to produce them. Just because it was produced in the past, doesn't mean it was free. You paid for it X years ago, and are only now seeing it used. You paid for it. Moreover, you're now going to pay to replace it.