Here's a summary/excerpt of the three ways of thinking through it:
1.) Bottoms-up ROI: We know everything and have put it in this spreadsheet
...if you have a good handle on the costs during some period of time where you were doing DevOps, and the gain that resulted from that period of time, you could come up with a bottoms-up ROI analysis. I think it'll be somewhat dicey since it's so hard to attribute costs and gains directly to DevOps but hey, it's better than either telling people they're asking the wrong question or its mute cousin: nothing.
2.) Were the efforts to change worth it? That is, DevOps is all cost
...if you want to run the numbers on something like "they tell me it will take three months and $50,000 in training and consultants to 'do the DevOps'" this might satisfy your ROI craving. Again, you'll need to have a pre-existing ROI at hand to simply plug your DevOps costs into.
3.) Pain avoidance and remediation
In the "DevOps is all cost" ROI scenario, we avoided ascribing gain to changing to DevOps. Again, while this is overly simplified, the deliverables of DevOps are to provide a continuous delivery process for your product and ensure that your product has excellent uptime. How could you account for the gain of those two desirables? You could create a way of assigning value to the knowledge you gain from weekly iterations about how to improve your product. You could also calculate the savings from avoided downtime.
Check it out, and tell me how you've been answering that question.