Governments encourage private sector participation in building infrastructure through Build-Operate-Transfer (BOT) agreements. Large projects may be financially non-viable despite their net economic benefits for the host society. Host governments might subsidize initial private investments to create financial feasibility. Small-scale subsidies might not sufficiently reduce project risks to attract private investment; however, large percentage subsidies might result in loose profit structures, discouraging the pursuit of efficiency. This article applies Monte Carlo techniques to data from the Taiwanese West Corridor High-Speed Rail Project to assess the subsidy-risk trade-off relationship. The results provide guidance for public-private negotiations.