I can't say anything about rent and|or hosting plans that utilize the Amazon Cloud.
But IIRC, the new EULA for ColdFusion 9 specifically mentions how the license applies to deploying on the Cloud. So it should be possible some way or the others. But I know nothing about the details.
I am planning a similar approach, moving ColdFusion to the Cloud on Amazon EC2. I need to be able to scale adding more web servers with CF on it.
Did you implementing it? any advices?
For scaling you can use elastic load balancer, add webservers or additional app servers. But it is important that you validate your architechture with your application first, depending on the traffic you are expecting.
In general its a good practice to use either relational or NoSQL database to handle authentication/client management. Ideally you should be looking at a seperate DB server for scalability and easier maintainance. You can use services provided by Amazon like RDS or of your choice (refer https://aws.amazon.com/running_databases/). However if the setup is small you can use the MySQL DB available with ColdFusion's AMI
I have Railo running in AWS (Elastic Beanstalk) and it works great. Just don't forget to enable Termination Protection or any changes you make will be blown away in the event of a service disruption.
Just got my bill for hosting in March (with an email server and MSSQL instance) . $1.61. One dollar and sixty one cents.