Hello Ingo,
we signed up to aws with the free tier and it's working. You don't have to worry about sizing as when you sign up for the solman 7.2 via cal.sap.com it automatically creates these instances in your aws. (and supposedly sizes them appropriately)
It creates 2x linux (abap+java) and one frontend windows.
We've used the Datacenter on USWest as it had lower charges then europe.
We've used it uhm... maybe around 10 Days (~9 hours each) in Feb and paid around $122 for AWS. (It's just to poke around and see what's new and stuff like that)
I can't seem to break it down anymore then that sorry. (just the value from the invoice)
Also we eventually did start the instances via the AWS console not via cal.sap.com as the latter starts all 3 instances but we we didn't needed the Windows Frontend (as we were using our SAPGUI and Browser) (also it saves you some moneys 
)
regards
Christian