> lost equipment costs basically 0 - they are mostly old hand-me-downs, the factories are not producing them anymore, so all the costs are already sunk;

Sure, for some legacy capital items, the production costs are sunk. But the cost of not having the equipment isn't. You can pay replacement cost or you can pay the permanent reduction in military capacity cost, but you are paying the cost either way if you lose the equipment.

Unless you are saying the operational value of the equipment is zero, in which case, sure, the loss is zero but then why did you even bring it to war in the first place?

> airplanes are more expensive, but again, this is old stock, there is plenty of them, and not incurring costs of producing more - let's say 10M per craft

If you aren't calculating it at the cost of replacing the capacity provided, you are underestimating the real cost.