Most SaaS products have close to zero marginal cost, since marginal cost is the additional cost of taking on one more customer. If you already have a thousand customers, what does customer number 1001 actually cost? In most cases, you do not need more infrastructure and you do not need more developers. Maybe you need a bit more support time to answer a few extra emails or take a few more calls, but that is usually it.

There are exceptions. Some SaaS products have infrastructure costs that scale linearly per customer, for example if you spin up a separate database for each tenant. Others are very hands-on, where a large part of the cost is human support or service work, so each new customer does add real cost.

But for the vast majority of self-serve SaaS products, the cost of adding one more customer is effectively near zero in the context of the overall business.