Just posted answer to a very similar question from another founder. There are a ton of ways to find data, commission studies, interview experts formerly employed by your competitors, etc that can be used to build models.
I personally, having done this type of work for many years, would never throw some numbers out without a basis for those numbers. Dig, dig, dig..you will find data that can at least provide a range of metrics that could/should go into a model.
Now, if you have a backer with deep pockets and he is not concerned about a model, or is willing to "launch and let's see what happens" please send me his phone number...;)