Simply put, if you require vast amounts of processing power, it's probably cheaper to just run your own data center because otherwise you end up paying for a cloud-based data center with all the third-party service overhead.
If, on the other hand, you simply require the equivalent of a few machines on the net, it's cheaper to buy a piece of cloud, because the extra per-CPU cost is small compared to the costs in maintenance, personnel, etc that you would have to keep in-house.
If, on the other hand, you simply require the equivalent of a few machines on the net, it's cheaper to buy a piece of cloud, because the extra per-CPU cost is small compared to the costs in maintenance, personnel, etc that you would have to keep in-house.