If an instance on-demand is, for the sake of easy discussion, $1.00/hr and the spot rate is usually $0.30/hr, it might make sense to bid $1.25/hr (or $1.26/hr) if you didn’t want to get pre-empted.

Most hours, you’d pay something close to $0.30/hr. Some hours, you’d pay over a $1.00/hr, but you’d save money overall against on-demand.

(This ignores Reserved Instances and Savings Plans.)