Our company audits shipping invoices and bills the customer based on a percentage of what we save them. It's a common business model outside of the tech/SaaS landscape (utility bills, property taxes, etc)
1. Customer original bill (variable - user provided) e.g. 100
2. Saving (variable - user provided) e.g. 9
3. Saving percentage (calculated) e.g. 9%
4. Billed Percentage (variable based on savings percentage?) e.g. 25% x 9
So you would have a billed percentage "rate card" that depends on variable factors like the customer bill plan, the amount/percentage saved etc? Is that the tricky bit?