Subscription and marketplace cases come down to one customer at a time.
- Contribution per month: price × gross margin. Use the margin, not the price: the price includes costs you pay to serve the customer.
- Lifetime: with a monthly churn rate c, the average customer stays about 1 / c months. 3% churn means about 33 months.
- LTV (lifetime value): contribution per month × lifetime.
- CAC (customer acquisition cost): marketing and sales spend / new customers won.
- Payback: CAC / contribution per month, in months.
A common rule of thumb: LTV of about three times CAC or more, and payback within a year or so. Treat these as prompts for questions, not laws. The usual traps are using revenue instead of margin, and churn quoted as a count rather than a rate.