Yes, but a huge reason to go with Amazon is to be able to take money back off the table. Sure, if you have predictably 24/7 load, then you may be better off racking your own at 150 machines. BUT, if you're not sure how much you'll need, and your app is built to scale up and down with demand, then that 150 number starts to slide upwards.

Risk mitigation is part of the cost of infrastructure that needs to be factored in, and if your business has a downturn, or your compute needs turn out to be lower, then you can scale down your OPEX pretty quickly. If you had bought them, you'd be stuck with them. (See Zynga)

The operating cost of doing it yourself is always hard to pin down, and include many many things. OS/Hypervisor licenses, staff, hardware, cooling, redundancy, disaster recovery, power, or Colo fees etc. The advantage with AWS, is that this just a few numbers on a few various services and it is very predictable.