It's marginal revenue attributable to a high-performing SRE (i.e. an SRE who would be able to elevate a product they're supporting from 90.0% availability to 99.0% availability.

It's actually a pretty high bar, because there aren't that many products for which the that segment of availability translates to >$1mm in marginal revenue. $1mm is a ballpark figure, but I think it's the right order of magnitude (i.e. the true number might be $5mm).

Expanding on another point in the original post: decision varies with the profitability of that marginal revenue. For example, it's basically pure profit for Google, Amazon or Netflix – accordingly, it makes sense that they'd have many people who focus exclusively on performance and availability, to make sure they aren't leaving that revenue on the ground.