With the ongoing open-access transformation, article processing charges (APCs) are gaining importance as the dominant business model for scientific open-access journals. This paper analyzes which factors determine the level of an APC by means of multivariate linear regression. With data from OpenAPC, APCs actually paid are explained by the following variables: (1) the "source normalized impact per paper" (SNIP), (2) whether the journal is open access or hybrid, (3) the publisher of the journal, (4) the subject area of the journal, and (5) the year. The results show that the journal's impact and the hybrid status are the most important factors for the level of APCs. However, the relationship between APC and SNIP is different for open-access journals and hybrid journals. The journal's impact is crucial for the level of APCs in open-access journals, whereas it little alters APCs for publications in hybrid-journals. This paper contributes to the emerging literature initiated by the "Pay It Forward"-study conducted at the University of California Libraries. It sets the foundations for the assessment whether the large-scale open-access transformation of scientific journals is a financially viable way for each research institution in general and universities in particular.