Ask HN: How to measure Product/Market Fit from marketing

My brother and I are working on marketing a new D2C app we built. My question is: based on marketing, how can we measure how good our product/market fit is? For example, if we spent $1000 on marketing, what kind of response should we expect if we have a good product/market fit? Another way to word this is: what should the customer acquisition cost be for a brand new D2C app (right off the starting line)?