I don’t think the analysis is that difficult. Imagine that the interstates were only used for military transportation (both for emergencies, which presumably haven’t happened much, and for regular logistics). Surely the ongoing maintenance costs would be vastly reduced.
Those costs would be the costs that “we the people” accept as valid usage of taxpayer money. You could throw in costs for other public interests like emergency logistics (e.g. FEMA moving goods and personnel for hurricane relief), first-class mail, etc., and you’re still probably only accounting for a very small portion of the taxpayer money that is currently going toward regular interstate maintenance.
The rest of the taxpayer costs are just subsidies of private interests, namely commercial freight and personal transportation. Those subsidies aren’t “free.” They come at the expense of other private interests, like freight and passenger railroads. These subsidies undoubtedly drastically change the economy, pushing everything toward just-in-time fulfillment, increasing greenhouse gas emissions, etc.
I would like to see these private subsidies reduced or eliminated, perhaps by charging tolls.