Depends on how bit the firm is. Very large consultancies with good lawyers who can commit to long-term support can do this and I have seen it. For example, one consultancy I was at charged "several pennies per billing statement" in exchange for re-doing the billing and statement system for a large telco. The consultancy also got a fixed fee upfront for initial investment.
This is sort of revenue sharing -- more customers, more statements (though not more services per same customer) means more billings
This is sort of revenue sharing -- more customers, more statements (though not more services per same customer) means more billings