CAC is the average cost to acquire a new customer. The formula is: total spent on marketing and sales over a period divided by the number of new customers won in that same period. It is one of the most important metrics for evaluating the efficiency of your marketing investments.
For SEO and Response Marketing, CAC tends to be lower than for paid media, because organic assets generate cumulative returns. A blog post that generates 5 leads per month for 12 months has a far lower acquisition cost than a Google Ads campaign that generates the same leads only while you keep paying.
Ideally, measure CAC by channel: what is the acquisition cost via organic, via paid, via social, via referral. This lets you allocate budget to the most efficient channels. Companies mature in Response Marketing typically have an organic CAC 3 to 5 times lower than their paid media CAC.