If you have a consistent level of traffic (i.e. you don't have inordinately wild upswings/downswings like e.g. Reddit), AWS isn't even remotely cost-effective. I was going to do the math to compare our current physical server infrastructure with AWS, and even if you factor in that physical servers need to be in pairs (for redundancy) and over-provisioned (for traffic spikes), I didn't even get as far as back-of-the-envelope math before it was obvious that AWS was completely infeasible.