Cost per lead is the easiest number to compare and the least useful one to manage. Two contractors can pay identical amounts per lead and end the quarter with completely different profit, because leads are an input and jobs are the output.
Start with your average job value and your close rate. If you sign one out of four quoted jobs and your average job is worth ten thousand dollars, every lead is worth roughly a quarter of a job. Multiply through and you have a rough ceiling for what a lead can cost before the math stops working.
Then subtract the costs that never appear on a lead invoice: drive time to estimates that go nowhere, the hours your office spends chasing people who never answer, and the discount you give when three other bids are already on the table.
This is why lead sources that look expensive per lead often look cheap per job. A lead you reach on the first call, quote without competition, and sign at your normal margin costs far less in total than a discounted lead you chased for two weeks.
Set up the tracking before you change sources. Record the source on every lead, the date of first contact, whether it turned into an appointment, whether it was quoted, and whether it signed. Sixty days of that data will tell you more than any pricing page.