The Customer Lifetime Value to Customer Acquisition (LTV:CAC) ratio measures the relationship between the lifetime value of a customer, and the cost of acquiring that customer. The metric is computed by dividing LTV by CAC. It is a signal of customer profitability, and of sales and marketing efficiency.
The ideal LTV:CAC ratio should be 3:1 or greater. The value of a customer should be three times more than the cost of acquiring them. If the ratio is close, you are spending too much.    
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 Startup Metrics You Need to Monitor, https://visible.vc/blog/startup-metrics/