Type their numbers. If the profit line is green, it's a no-brainer. If it's red, walk. Pull this up on the call and let the math do the closing.
On the call: "Each customer's worth about $[value] to you. If I get you customers for $[cost] each and you pay me $[fee]/mo, you spend $[total] and get $[revenue] back." Then stop talking. The green number closes it.