> PLOS ONE’s fee is $1,595. A factor of 3 doesn’t seem inexplainable to me.

You need to first explain why PLOS ONE is an appropriate baseline. A world class Open Access journal costs roughly $10 per submission [0]. Most arguments I have seen using PLOS ONE as a baseline, talk about the "non profit" part of PLOS. It has to be stressed here at that "non profit" doesn't mean PLOS works on a "non profit" business model. It just means that they generate profit AND the profit isn't distributed to its members, directors or officers [1].

[0] https://gowers.wordpress.com/2016/03/01/discrete-analysis-la...

[1] https://www.law.cornell.edu/wex/non-profit_organizations