Tonight I played a little bit with EC2 from Amazon. The good news is that you can use RS web app with EC2 Micro On-Demand Instances. And those are limited, but cost only $0.02 per hour which is just $15 a month. (comparable with jiffybox in Germany for 0.02€ a hour).
If you create an EC2 instance, you can choose Ubuntu 12.04.1 LTS in 32 bit. This way you already have all the required libraries preinstalled for Real Studio Web Apps.
I think this is a great way for Real Studio developers. Such an instance is setup in 5 minutes, you connect with SSH, upload your app and launch it!