How much does it cost the publisher to publish an academic article?
Ubiquity Press breaks down their £300 ($500) APC as follows:




38% indirect costs for things not related to the publishing of a single paper but which are needed for the business (£114 or $190)
34% covers editorial and production aspects, which appears to be the costs associated with producing the paper, managing submissions, responding to authors, preparing proofs, typesetting, XML etc. (£102 or $170)
16% is a waiver premium charged so they can offer 0 or low APCs to people who genuinely cannot pay (£48 or $80)
8% is used to pay for indexing, archival (in case they go bust), DOI etc (£24 or $40)
4% goes towards costs of billing you and taking payment (£12 or $20)


Depending on what you consider to be the actual publishing costs (here probably the 34% editorial &amp; production costs + 8% Indexing &amp; Archiving) you would be looking at ~ £126 or $210.

Ubiquity don't break their indirect costs down into server/platform costs; this all goes into the 38% indirect cost column.