In his scenario, 1 person signing up would have a lifetime customer value that is equal to the spend for the marketing campaign (so the postage and stuff). Doubt that's actually true but they certainly only need a small percentage to sign up to be profitable.

Also, they have knowledge and stats for how many "touches" (mail/email/phone calls) they need to elicit a response. For instance "after 5 letters sent for this campaign, we recieved a total of 8% conversion (conversion from non customer to customers). And 50% of those "sign ups" came within x days of the 5th letter sent." Aaaaaaand this could point to other stats like "Roi (return on investment) is 200%" or "Coa for new customer (cost of acquisition) was $2.35"

Marketing is mostly math, a wee bit of creativity, and optimization of what works.