Let's say that, for a unit of computing work, ,the cloud price is N times of non-cloud. I'm being a bit vague here for generality; plus, when not on a cloud, the cost structure is different, but bear with me.
Ok. Now, your load varies by a factor of up to 133x. But unless your peak load is over N times your average, it is still cheaper for you to keep machines which support the peak load and have a bunch of idle time.
Extra benefits:
* Can do other computational work (e.g. experiments) during off hours without impacting system responsiveness.
* Can perhaps put some machines to sleep, or other power-saving measures, during off hours.
Extra detriments:
* You have to take care of more system and cluster administration work than on the cloud.
Above N, the cloud makes sense. Close to N - not sure. Well below N - doesn't seem to make sense.
You'll need to tell us that N is low enough.