Will someone who has been successful with charging royalties for a sales letter give us brief over view..."How Do You Charge Royalties" for a sales letter.

All the marketing books talk about doing it, but I have yet come across "How To Make It Happen"?

Example: Lets say I charge $200.00 for a sales letter and then want to charge royalties...how do we know what the customer is making as a direct result from this sales letter and know what to charge them and do you have them pay you monthly, yearly?

Thank you,

Dave