Datapoints motivating introduction of SRE in organisation
The types of numbers you're looking for might be hard to come across, because they're highly variable (even within one organization, it varies service-to-service and team-to-team, in my experience.) The SRE Workbook is now available for free, and includes two case studies (chapter 3) that might be helpful. Also, New Relic's SRE eBook does a really good job of summarizing SRE in a concise way.

Another way to approach this would be to try to use what you know about your service today to create a risk assessment and estimate downtime you can prevent if you had SRE and dev support to eliminate those risks