There is quite a lot of software (mostly in B2B market) that has a very high marginal cost, where the cost of doing sales and customer-specific customization can easily dominate the fixed cost of making the pre-sale software.
- if you end up with a recurring license afterwards (which is what were're talking about here because SaaS works this way) then it's not even marginal cost, it's a fixed up front cost and then you have a stream of revenue. You know it's fixed costs because you don't have to pay it again when you're billing the customer for the second year. Same if your license includes a per-seat/per-CPU price.
And the problem with the idea of “marginal cost” is that in practice almost every cost is a fixed cost.
For instance, let say you have two guys that are dedicated to the customization of your product per client. You call this customization “marginal cost” but in reality it's fixed cost, as their salary is due whether or not you have a customer's product to customize this week.
And I'm not being original here, as I said it's straight from the work of 1988 econ Nobel prize Maurice Allais.
Customer acquisition cost at least ticks the box of being a net expense, but it's still a one time cost.