Most people and/or small business owners aren't familiar w/Progressive Web Apps. Due to the unfamiliarity of PWAs, they are difficult to explain what powerful tools they can be for various industries and organizations.
Are they Guaranteeing you revenues of a particular sum then you can.Else Bargain on the revenues generated. Slabs for every slab of revenue generated.
You can use the main benefits that Progressive Web Apps inherently possess to help provide the value. Since search has gone almost exclusively mobile, use the fact the PWA's are mobile-friendly and responsive.
A comparison point to bring up with SMB Owners could be a demonstration of their analytics of Traditional WebSite Benchmark versus the PWA Benchmark for a similar business site.
Explaining the concept while stressing the need for mobile-friendly responsive ways to reach their customers while showing them a real life case study example should work.