If you have the requirements for more than a few VM's on AWS, then the pricing for physical is going to be cheaper by far, even with a three-stack of physical servers. Let's put a basic Poweredge R620 as an example. For about $10k/server (and I'm going for buying hardware to kind of do a worst-case-scenario), you can get 384GB of memory per server, times three that's over 1T of memory in your cluster.
Given the EC2 Small instance type of 1.7GB/VM that would be over 600 VM's you could run on that kind of stack. Pricing for a half cabinet with power and bandwidth (you can find places with un-metered gigabit connectivity) will run you no more than about $2500/month (Much less if you are good at negotiating). So you have an outlay of around $40k for total hardware costs and recurring monthly of $2500/month. Amortizing this over expected lifespan of three years for the servers (worst case scenario), you can estimate per month costs somewhere in the range of $3600/month.
With EC2 to reach parity with those 600 'Small' instances would be an initial deposit of $57,600 (for three year term) and a recurring monthly of $11,826 for a total 'recurring' of $13,426/month.
Now, I should point out that these are obviously not apples-to-apples if you have: Huge CPU requirements on those 600 instances, or need lots of disk. Your up-front hardware costs increase (perhaps substantially) once those come into play. More I'm just trying to compare a bare instance scenario. As per one of my comments elsewhere in the thread really it all comes down to workloads and doing a real business analysis.