[nullc]

At a minimum you should know what you're getting into.

Imagine that that server is controlled by your worst enemy.

Still okay with using it? If so thats good evidence that you can just go ahead. If not, ... well, it gets harder because how do you know your worst enemy doesn't or won't gain control of it? (What? your worst enemy is the neighbour's chihuahua and Amazon doesn't hire dogs?)

WRT the costs there, I'm not sure that really follows. At least on the high end EC2 is phenomenally expensive if your load is predictable, it's interesting when you can't predict it. The large high cpu instances basically pay for the hardware in a couple months worth of use. Small instances are low performing to the point that a $35 quad core arm or $50 atom device is competitive. Surely there are cases where EC2 is cheaper, but "not computing at all" sounds like an exaggeration.

[Turing_Machine]

"Small instances are low performing to the point that a $35 quad core arm or $50 atom device is competitive"

You're forgetting the "Amazon data center connectivity", "can scale up or down at will", and "someone else's problem if the hardware fails" parts. Those things are not free.

[ldng]

Yes but sometimes cheaper than cloud provider make you think.

IMHO, it' a bell curve. Before a certain point and past another threshold the "cloud" is more expansive than it's worth. Adapting your application/system to a IaaS or a PaaS model isn't free either.