I just used my regular Amazon account to sign up for EC2. It's currently running on the free tier. It's only free for a year, however.

Version 3 of the Unifi firmware allows for multi-tenant installs. So, you would be able to monitor multiple sites using one instance of EC2 or whatever your controller is running on. When I was setting this up, 2.9x was the current stable firmware, so that's what I went with.

You can run as many controllers as you want with with Amazon. Only the first one will be free if you want to keep them all running under the same Amazon account. I don't know what the cost will be per month yet, but forum anecdotes seem to calculate it at $7-12 per month for a small install such as what I did. The cost will vary based on how much data is sent from the APs to the controller in Amazon's cloud. More APs on-site, and the price will go up.