Kudos to BioMedCentral for this explanation of why there is no APC (yet) for some of their journals: “† No article processing charge is payable for this newly-launched journal, for a promotional period.” from: How much is BioMedCentral charging? (viewed Aug. 1, 2014).
This is a straightforward explanation of a very common business model (for many kinds of businesses, not just OA journals) of free or low cost introductory pricing, the purpose of which is to attract customers with a view to eventually instating different and quite possibly substantially higher pricing. This is most important to keep in mind when projecting potential future costs, as the low costs of commercial for-profit open access publishers today may be substantially less than what some intent to charge after their journals become successful.
This is not meant to suggest that this is the practice for all open access journals and publishers using the APC method, or even most, rather it is one approach to be aware of that the second-largest publisher using this method is rather helpfully telling us it is a method they consciously employ.
Morrison, H. (2015). OA APCs: Beware introductory pricing. Sustaining the Knowledge Commons / Soutenir Les Savoirs Communs. Retrieved from https://sustainingknowledgecommons.org/2015/06/20/oa-apcs-beware-introductory-pricing/