I imagine the threshold is something like 1 SRE for every $1mm of high-margin revenue you can link to guaranteeing the 2nd "9" of $product availability/reliability.
I believe that is indeed a good guide for when it makes sense to have a SRE team supporting a service or product (with the caveat that the number probably isn't $1MM).
There are also good patterns for ensuring you actually have adequate SRE coverage for what the business needs. 2 x 6ppl teams geo-graphically dispersed doing 7x12 shifts works pretty well (not cheap). You can do it with less but you run into more challenges when individuals leave / get burnt out / etc.
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.
There are also good patterns for ensuring you actually have adequate SRE coverage for what the business needs. 2 x 6ppl teams geo-graphically dispersed doing 7x12 shifts works pretty well (not cheap). You can do it with less but you run into more challenges when individuals leave / get burnt out / etc.