The pricing of government guarantees is an actual subject and it is solved with the usage of Real Options Theory. Usually, these minimum demand or revenue guarantees are modeled as PUTs where the concessionaire is long and the government is short and it is calculates its value added gain to the project. However, almost no discussions are made to the long term liabilities that these guarantees can bring to public finance. In this study we analyze the second bidding of the Rio – Sao Paulo High Speed Railway project, which with a total investment of 37 billion reais (18 billion dollars) was the largest PPP auction ever held in Brazil. We show that only the addition of government guarantees makes the project feasible and we price 12 different levels of guarantees. We use VaR and CVaR as tools to measure the liability risk that government is exposure as it short in these PUTs. In order to mitigate these risks, we add 10 levels of caps in order to limit the sum of the PUT values in the 40 years of the project. In the end, we created an objective function in order to define the best combination of minimum guarantees and government liability caps in order to maximize the value of the project value for the concessionaire and minimize the risk for the government. We believe that is the best combination for the society.