Abstract: In the current article, we tested our hypothesis by which high-impact journals tend to have higher Article Processing Charges (APCs) by comparing journal IF metrics with the OA publishing fees they charge. Our study engaged with both journals in Science, Technology, Engineering and Mathematics (STEM) fields and the Humanities and Social Sciences (HSS) and included Hybrid, Diamond and No OA journals. The overall findings demonstrate a positive relationship between APCs and journals with high IF for two of the subject areas we examined but not for the third, which could be mediated by the characteristics and market environment of the publishers. We also found significant differences between the analysed research fields in terms of APC policies, as well as differences in the relationship between APCs and the IF across periodicals. The study and analysis conducted reinforces our concerns that Hybrid OA models are likely to perpetuate inequalities in knowledge production.