As a service, I'd scope this around helping clients prepare knowledgeable replies that they review and publish themselves, using real examples from their business. That is a clearer offer than selling a fixed number of comments.
For a pilot, agree on the audience and track relevant profile visits where measurable, conversations and enquiries, alongside the hours needed to research and respond. A post getting more comments is easy to count; whether those comments produce useful business conversations is the part I'd want to validate before calling it a big opportunity.
