Thanks for all of the advice everyone, especially the bits about starting larger, which is something I tend to always skew towards anyway, dreading the possibility that the text will be less legible when shrunk. My client at least knows that they want it one page portrait orientation, which yeah of course is the first hurdle to overcome.

Especially because some authors are in the process of writing when they commission a map, what they really want is, something they can reference while they work. So I always suggest my A4 pricing, and make sure the text is at a scale that reads for the final print size, but the map can be printed larger without blurring.
This is my case exactly Tiana - at the outset of this project it's mostly for their reference. And it's a good thing they had a cartographer on hand to go over it too; we wound up reducing the size of the landmass they wanted to depict by quite a lot based on what they'd originally said the scale was.

This isn't something that anyone necessarily needs to give specifics on if you don't want, but given a map request where you know the general density of the features the client wants, what's your general price range when you quote them a few options, like one quote a one-page portrait, another quote a two-facing-pages, another for a larger version, etc? For example, if you in your mind use a one-page portrait orientation of the client's map as your "base" quote, is a two-facing-pages map like, 150% of the base cost, 200%...