I'm having a similar problem, mainly with revenue projections with advertising.
So I'm just building out a table - each row in the table is some level of traffic, and each column has some projection of yield (yield = CTR X CPC), like reasonable worst case, probable case, and reasonable best case.
Basically, identify the variables in the model. Come up with some reasonable ranges for what those could be, put those in a table. For example, for CPC advertisements, the CTR that I have heard about range from a minimum .05% to a maximum of 30%, and the CPC ranges from $.01 to $100 (e.g. for mesothelioma related ads at one point).