I always say something like, "here is your project evaluation, based the current desires/spec for your project. Certainly things can be add, removed, re-prioritized. When work begins this should be seen as a living document that grows and evolves with the project."

The initial evaluation will help you fill out your client's backlog, and also understand how many people (with what skillets) you need on the project.

So, think of it as getting paid to do Agile Project Manager work. At least how I see it. :-)