There are two ways to value a trail book and they should land close together. The multiple method takes annualised trail and multiplies it by a market figure, usually between 2.5× and 3.5× in Australia. The discounted cash flow method projects the trail year by year, shrinks it by the expected run-off, and discounts the result back to today at a rate that reflects the risk. A DCF at a 20% run-off and a sensible discount rate produces something close to 3×, which is why the shorthand works.
Both methods depend on the same inputs: the trail actually received (not the lender's gross), the run-off history, how much of the book is inside its clawback window, the lender mix, and arrears. Buyers ask for at least twelve months of commission statements to verify all of it, and many ask for two to four years.
A valuation is only as good as the data behind it. Brokers who reconcile every statement can show a buyer exactly which loans earn what, and that transparency is worth a quarter turn of the multiple on its own.