The following is all based on how much it would cost to 'do it yourself' i.e. coordinate every last aspect through to print.
Best case scenario: Zero. But that's finance we're talking. It's still going to cost you time and energy. You'll have to think about how you'll get a good (emphasis there) book jacket designed, do all your own conversion and upload to various etailers. You don't need an ISBN necessarily (or you can publish through Createspace which has ISBNs and then lists itself as the publisher). You will need to edit, edit, edit then proofread and check your final documents before sale.
Spending some money scenario: I don't want to call this worst case, because I don't think it is and I also think it's best to break down the figures into where the money is spent rather than give a lump sum. This is also UK specific, with a slight dollar cross over. So:
+Buy ISBNs: £118.68 (You must buy in a block of 10 and it takes no more than 30 minutes to register a publishing company with Nielsen - form here.)
+Buy/register a copyright: up to £84 for life (Although you automatically have copyright to anything you create anyway this is just some extra legal proof should you need it.)
+Get book jacket designed: I've seen prices range from $40-$120. (Try checking out Etsy for graphic designers or check out the designers of covers you admire. A note of caution: buy the rights not just a one time usage or a license. You want to pay for this once and once only.)
+Editing fee: the sky is your limit on this one, but make sure they have book editing experience rather than riding on a hopeful aspiration that they'll make a good editor. And that's actually editing, not just a really enthusiastic reader (though beta readers are, no doubt, really handy too). You want someone who can enforce consistentcy and will make a good critique that helps you improve your work. (Or consider going pro-bono and swapping your work with a crit partners - not always easy or viable but is another spend-free option.)
+Proofreading: expect £1-£2 a page, max. I see very few books that are 100% perfect. There is always something wrong with them whether it's a speech mark that's the wrong way round, an incorrect homophone or a missing connective. Heck, I've even let stuff through to press with a spelling error - there's probably still errors on this page. You need someone absolutely hot on grammar, spelling and punctuation (and preferably from your home country given English language variations) to sharpen these up because readers do notice. And a good proofreader is really, really hard to find.
+An accountant: yes, one of them because you're going to have to declare tax at some point and it's a bother if you're not familiar with filing self-employment taxes.
Anything I've missed? Chip in...