Short definition
Cost per lead is total marketing spend divided by the number of qualified leads produced, the headline efficiency metric for any lead-gen channel.
In depth
CPL is the cleanest comparable metric across channels. Calculate it monthly per channel and per campaign. For service businesses the meaningful CPL is qualified — strip out tire-kickers, bots, and out-of-area leads. Healthy CPL varies wildly by industry: a roofing lead might cost $80–$200, a med spa consult $30–$90, a custom software inquiry $300+. Track CPL alongside close rate to derive cost per acquired customer (CAC) which is the number that actually matters.
Example
A landscaper sees Google Ads CPL of $95 and Facebook CPL of $18, but Facebook leads close at 4% vs Google at 28%. True CAC: Google $339, Facebook $450.