You're right in that the cases of real-world examples are rare, but I believe it's only in software. The concept of outcome-based pricing have always existed throughout different work for a long, long time – think about test-prep service that promises to only need to pay if you get X score, real-estate agent that carry commission as a % of sale price, etc.
I think this instills confidence in something like this and makes me wonder why such pricing models haven't been applied to tech yet.