The time cost is real. Referral partners need lunches and follow up. Past clients need a reminder before their bank sends one. Ranking for your own name needs a website that says what you do and where. None of it is hard, and none of it can be scheduled, which is the problem when you need settlements this quarter.
Free leads also arrive unverified. Nobody has checked the phone number, run a duplicate check or screened the person on finance, so the qualifying work is yours. Paying trades money for that work and for a predictable flow, which is the case for outsourced lead generation for mortgage brokers. How brokers combine the routes is covered under how to generate leads as a mortgage broker.