What it is
An output metric, not an input.
Cost per lead is what happens after CPC, conversion rate, and attribution have done their work. You do not set it. You inherit it.
Every cost-per-lead figure is the compound result of two upstream variables: the cost of the click and the rate at which that click converts. The arithmetic is inescapable — a $5 CPC at a 2% conversion rate produces a $250 CPL, and a $3.50 CPC at a 4% conversion rate produces an $87.50 CPL. There is no third variable, and no negotiation with an agency or platform will move CPL without moving one of the two inputs.
This is why operators who ask “how do I lower my CPL” are asking the wrong question. The real question is always “which of the two inputs is broken, and by how much.” A diagnostic audit answers that. A request to “reduce CPL” without specifying which input to work on tends to produce bid cuts and campaign pauses — actions that reduce spend faster than they reduce CPL, and usually lose more leads than they save dollars.