I think you could get 75 of Dell R410s with 8 cores each and 16G of memory for under $300k. A top of the line colo will run you about $2k a month for a rack and a half with power and room to run all 75 boxes. Where you run into trouble is if the company you buy from charges you for the box cost every year. Getting charged as if you are re-buying every year would work out to something like $27k a month. Those boxes will easily last 3 years and if you span it out over 3 years you are looking at $10k a month. That is half what you are paying for EC2.
Yes, the dedicated servers might be less. But when one of them breaks, I have to wait for the provider to fix it. On EC2, I can replace it in 5 minutes.
With 75 good quality machines I bet you wouldn't see more than three hardware failure a year if that. All 3 of those would probably be drive failures. I imagine the redundancy of the software would handle that without a problem.
I agree that the continence of EC2 is very nice. We use EC2 a lot but just in a lot more "elastic" way. We have found it is cheaper to colo boxes if the demand is constant.