The openCost-project is funded by the German Research Foundation and was launched by the three project partners Bielefeld University Library, University Library Regensburg and DESY Central Library in 2022. The project's main goal is to promote standardized collecting of publication cost data in order to improve the transparency of costs within institutions and also enable inter-institutional cost comparisons. This kind of data includes publication-related fees like APCs or submission fees, but also costs from transformative OA agreements or memberships. The basic idea is to integrate financial data directly into repositories on record level, making it accessible together with the existing bibliographic metadata. For this purpose, a metadata schema was developed to comprehensively store and exchange data. This XML schema and how the partner institutions utilize it to enrich their repositories with publication cost data will be presented. An open interface for harvesting and querying via OAI-PMH is to be implemented and realized exemplarily on the publication servers of the universities of Bielefeld, Regensburg as well as DESY and its partner institutions of the shared repository infrastructure JOIN². Via OAI-PMH, service providers (e.g. aggregators and research funding agencies) can harvest data directly from the institutions. Within the project’s framework, this is demonstrated by the OpenAPC service (Bielefeld). By implementing a technical interface, the services of the Electronic Journals Library EZB (Regensburg) will also be extended to include detailed information on OA publication costs, to be finally used as a central information platform for communicating OA information to researchers. Overall, the development of the openCost standard is carried out with the involvement of national and international expertise, including continuous public discussion and feedback rounds on the agenda. Accordingly, openCost lives open science from the idea through the implementation phase to the target of comprehensive cost transparency in the scientific publication process.