spot

Spot is 60% cheaper until the preemptions eat the discount. The expected-loss formula (waste per preemption = half the checkpoint interval plus restart), the break-even worked at three cadences, the checkpoint cost that pushes back the other way, and the gang arithmetic that says which jobs must never be on spot.

Any engineer can cut cost. The question is whether you measured what it cost you elsewhere, and whether you knew that before you shipped it. The three mechanisms that produce most of the savings, and the metric you have to watch on the way down.