I believe that the CAC (customer acquisition cost) is not a relevant measure for a marketplace. It is relevant for a recurring-payment SaaS product.

I suggest finding a more meaningful measure for a marketplace, e.g. Cost per Sale in Marketplace (CSM?).

Then you can measure the average sale price in the marketplace, the average commission your business takes, and go from there estimating the required cost per sale in the marketplace to make it worthwhile.

For example, if your sales transaction price in the marketplace averages to $100, and you take $5, you need to make sure that the cost of making that happen is not more than $5 (and preferably much less).

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