Ask HN: Cost structure of SaaS company

I am about to make a presentation to investors and thus looking for some authoritive numbers to calculate and proove my financial projections.

Does anybody have information/links about the typical cost structure of a SaaS company: how much needs to be calculated per visitor/prospect/customer for support, marketing, acquiry, infrastructure, ... (did I forget something important)?

And is there a way to build in the positive effect of word-of-mouth in those projections?