Article processing charges (APCs) vary considerably and in a number of disciplines some peer-reviewed OA journals don't levy APCs, i.e. costs associated with publishing the journal are subsidised by a professional association or institution.
Shamash, K. (2016). Article processing charges (APCs) and subscriptions: Monitoring open access costs. Retrieved from
https://www.jisc.ac.uk/reports/apcs-and-subscriptions

** based on data collected from 70 higher education institutions across the UK.