If the actual cost structure was better known, it would be a lot easier to evaluate if the price level is reasonable. Also, it would give a push towards "unbundling" most of the APC. Eg, the author could do the typesetting themselves (as in JMLR) or buy it from somewhere else for cheaper, and thus save grant money that could be used to pay at least something to a grad student or a postdoc currently working for free instead of, as I cynically assume, line up pockets of rich investors and overpaid execs.

I find it a very odd claim that somehow doing publishing at large scale should be MORE expensive than doing it on small scale. My hypothesis is that it's not the production costs, but that at the moment academic publication is so ridden with network effects that publishers can ask for greatly inflated prices to make huge profits and/or be done very inefficiently. It's also such a good business that journals/smaller publishers tend to get bought out or at least invested into by larger companies (most of this was actually done in past couple of decades), who of course have an interest to keep the profit margins as high as possible. And this can happen quite naturally even without full monopoly or cartel, although I wouldn't be surprised if there's some kind of "common understanding" among the big publishers that price wars aren't good for any of them. Also, it's one of those businesses where almost nobody actually pays anything from their own pocket directly, which generally isn't very good for price competition.

But in the end, I find most of this discussion sort of moot. I quite strongly believe, and can't seem to get any evidence to the contrary, that the actual labor going into the publishing is so low that the industry in it current form shouldn't be, and hopefully won't be, sustainable given the current, let alone future, technology. But OTOH, even if I'm correct, they have huge lobbying budgets, democracy is in such a sorry state and people are so indoctrinated to be selfish, short sighted greedy creatures, I wouldn't be too surprised that the industry can keep the racket going for decades.

But yes, prices are not a reflection of cost, but of how much publishers can ask. Unfortunately, given that publishers sell subscriptions in bulk ("Big Deals"), there's no downwards price pressure: if they add another journal to the deal, they can ask more money, without that leading to libraries cutting subscriptions elsewhere. There's no real pressure for APCs as well: authors want to publish in "reputable" journals, and don't pay the APCs themselves, so publishers are free to raise them quite considerably.