Before we design anything, we draw the whole site as a tree: Home at the top, the main pages under it, and their sub-pages under those (sample note).
Each box gets a name and a job. If a page has no job, it comes off the sheet. Clients are often surprised how many pages of their old site nobody needs.
Once the sitemap is approved, the price is fixed. During the project we shade each box as it moves from wireframe to design, built and live, so everyone can see where things are.