Everything you spend to land one new paying customer in CAC (Customer Acquisition Cost), fully loaded across ad budget, sales team comp, and the tools they run on.
Dropbox famously cut blended CAC dramatically in 2009 by replacing paid acquisition with a two-sided referral program offering 500MB of free storage to both inviter and invitee. The program drove signups from 100K to 4M in 15 months, a case study now taught in every growth class as proof that product-led referrals can collapse CAC almost to zero.
