Writing in The Guardian, Suzanne McGee tallies up some of the costs involved in self-publishing and comes to a figure of $6,000 per book.
It probably goes without saying this figure is, um, controversial in self-publishing circles, and many people manage to get things out there for a hell of a lot less (say, between $0 and $6,000 less), either by doing things themselves or by trading favours in the community (“I’ll do your editing if you do my cover art”). So it’s worth taking the number with a grain of salt.
What the post does illustrate, however, is the sorts of costs involved: ISBN, editing, cover art, marketing, etc. How much or little you spend in each category is up to you–that’s, yanno, sort of the entire point of self-publishing–but it’s good to at least have an overview of what the line items on your budget should be, even if there’s only zeros in the right-hand column.