Public passenger transport represents a fundamental element of regional mobility, ensuring accessibility and social inclusion across all areas. Its effective and transparent financing is a key condition for maintaining the long-term stability and sustainability of the transport system. Long-term public service obligation contracts are, however, significantly influenced by fluctuations in fuel prices and labour costs, which creates uncertainty for both transport operators and public authorities. Therefore, it is necessary to establish an objective and predictable mechanism for price adjustment. The aim of this study was to design and verify a methodology for adjusting prices in PSO contracts using reliable statistical indicators published by national statistical offices. The research process consisted of five stages: analysis of existing contracts, identification of key cost components, development of an adjustment model, testing on historical data from 2016 to 2024, and evaluation of the model’s accuracy. The results confirmed that the differences between calculated and actual unit costs were within ±0.01 to ±0.02 € per km, representing a deviation below 5%. Consequently, the proposed model can be considered a reliable and transparent tool for price adjustment in long-term public transport contracts.