Open access publication often entails payment of Article Processing Charges (APCs), particularly in the so-called Hybrid and Gold journals. The growth of Gold OA publications linked to the development of open access mandates has forced funding institutions to take over APCs. Thus, this research seeks to quantify the amount of public investment dedicated to APC within the projects funded by the Spanish State Plan for the Generation of Knowledge and Scientific and Technological Strengthening of the R&D&I (SNP), the main public research funding instrument in Spain. The period studied is 2013-2018. The preliminary results show that 20,399 million € were spent on APCs, accounting for 1.5% of the total budget funded by the SNP. In addition, the average percentage of the budget dedicated to cover APCs has been calculated for a sample of projects. Most research areas are dedicated between 0-5% of their budget to cover APCs. However, numerous outliers with rates over 10% suggest further study on the role of APCs in the financial performance of the research activity.