BMC has a series of journals that publishes on maternal and child health. It is a good impact factor journal and it gives a full waiver if the study is not funded.
Seems to me depends on the availability of the reviewers which often the author of the paper submits with the manuscript. When you say "costs", are you referring to the APC (article processing charge)?
It is my understanding that journals which make the publication contents available to anyone who is interested, collect revenues from the authors/researchers whose work is published in their products. Articles published in journals which do not apply a publishing fee --% rates of acceptance are detailed on publication websites- do not generate income for journals. Journals get their revenues from APC's. Not all journals which publish articles collecting APC's apply PMID's to articles.. so researchers have a harder time accessing them. Hope this help.