If you just write for yourself, it only costs time. If you plan to (*self-) publish it, though, you want at least a good cover, and optimally, you’d hire an editor and maybe things like sensitivity readers. And then, most people seem to prefer audio books these days, which is either expensive, or hard to pull off, due to having to find a narrator who’s okay with royalty share with a non-established author. And then you haven’t advertised your book at all yet.
I’ve so far only worried about cover and editing. Wrote 4 novels. Now I’m writing a series and am considering writing the whole thing completely first, then getting a deal with an artist for all the covers. This also makes it easier to do foreshadowing properly over the course of more than one book, and it’s probably advantageous to stagger book releases, even if that means a few years without putting anything out to the world.
*All these points are moot if you aim to get published by an established house, but then you’re dealing with “the suits”, and people who rank “will it sell” higher than “is it good”.