Customer acquisition cost, or CAC, is the total cost of acquiring a customer: ad spend, agency or staff cost, tools, and creative, divided by customers won. Most teams undercount it by leaving out everything except media.
CAC only makes sense against customer value. A high CAC can be healthy if customers are worth far more over their lifetime. The ratio of CAC to value, not CAC alone, tells you whether to scale or fix the funnel first.