Like I said, it's 100% free to developers. It's like an extension of the box itself... You can just use it without fee, or charge.
That said, there is a knowledge-base and expertise level that comes with taking advantage of it... You need to consider scalability and resilience testing, etc...
Like I said, there will be justification for not using it for all titles out the gate, but as time goes on and expertise improves - we should expect to see many games use cloud compute for all kinds of things... Not just dedicated server-like functionality, but things like drivatars, AI, persistent worlds (even in SP games), etc...