CPL is the cost per raw lead (e.g., form fill); CAC is the total cost per acquired client. CAC is always higher than CPL and depends on lead-to-client conversion rate.
Cost per lead (CPL) and customer acquisition cost (CAC) are often confused. CPL is simply ad spend divided by number of leads generated. CAC is ad spend (plus sales cost) divided by number of clients actually enrolled. If CPL is £100 and lead-to-client conversion is 10%, CAC is £1,000. CAC is the only metric that matters for profitability.