If the government wants to change the logo on the login screen, they're gonna pay 1 person-year. Why would I quote less? It'll take over 1 person-year for another contractor to pick up and start supporting the codebase, and we won't support a codebase that another contractor has touched.
It's just the nature of government IT. Lowball the initial quote, then charge massively for any modifications and support now they're locked in.