Previously submission by the researcher and subsequent publicaiton is effectively free; the real cost is paid by subscribers such as university libraries as well as commercial entities.
Under the open-access model the researcher pays the publisher directly for each submission and this can vary from hundreds to thousands of dollars. PLoS one currently charges US$1495 for each publication.
https://www.plos.org/publications/publication-fees/
The exact burden depends on the subject, STEM may be better funded, but this is still not a sum that one can just shrug off. Often grant money also come with strict stipulations on exactly what expenses can be paid with them - publication fee is usually not provided.