Military costs upkeep, so a commie that keeps lots of military on hand loses cash each turn.

I calculated it out once. Let's say you have one jet, and there are 1000 turns left in the set.

You are better to sell that jet for $100 and buy a jet later for $180, rather than holding the jet for 1000 turns and paying upkeep.

I think the breakeven point is around 500 turns - you're better off keeping the jet for 500 turns, if prices are around $100.