I’m willing to be pretty flexible really. Setting too hard a limit is going to lead to too much nitpicking. I’d call a reasonable $21 target okay – like if that last handful of tanks puts you over but makes your ToE make sense, then go for it. But a full extra tank battalion when you were already at $20, that would be poor sportsmanship.
We’re all wargamers here – I think we could all be comfortable within the constraints of the honor system.