CPL (Cost Per Lead) is the average cost of one lead: divide ad spend by the number of leads received. Many judge marketing by this number and assume "cheaper is better."
In reality, the most expensive thing is a cheap lead. A low CPL often means low-quality leads: they fill the sales team with empty calls and raise the real CAC.
An expensive lead is sometimes more profitable
On some projects a deliberately expensive, filtered lead is the right call: a 10-question filter form may push CPL to ~$10, yet 90% of the leads may be sales-ready. On another project the opposite holds — a cheap, simple form works better.
The numbers, not guesses, decide which is right: run several funnels in parallel, calculate CAC for each, and pick the winner accordingly.