Not defending it but based on descriptions I’ve heard about government air gapped networks I wonder if it’s less about cost of individual computers and more about the annoyance, overhead and expense of the transfer systems.
Yeah there is definitely annoyance, overhead, and expense but you are trying to securely transfer data to an air-gapped network so just "plugging things in" (what some people expect, like just grabbing a usb drive) is a risk in that regard so it's a lot of trade-offs.
