There are three models for publishing an ebook:
1. Traditional publisher, hard copy and ebook. You place your book with a traditional publisher who issues a hardover or paperback version as well as an ebook. Royalties on ebooks done this way vary but should be at least 25%. The advantage is that there's a chance your books will get into book stores--still important despite the diminishing number of book shops. However, don't expect a big marketing campaign for your book. You'll get good marketing support for a week to a month, after that you're usually pretty much on your own. Publishers are being more selective these days, so getting your manuscript accepted is not easy.
2. Ebook publisher, ebook only. The publisher does the editing, cover design, formatting, etc. and markets the book in a variety of ways and pays you a royalty. Again, the percentages of your royalty vary, I've seen from 20% to 50%. The advantage is that you get all the services mentioned, and distribution. Marketing? Same situation as above.
3. Self-publish an ebook, pay for services with a one-time fee. In this version you outsource editing, cover design, formatting, submission to the various sellers like Kindle, and keep the money you make from sales. The advantage is that if your book sells well you'll make more money than if you're getting just a royalty. However, you'll end up doing all the marketing yourself unless you outsource that, too--which can be expensive and there aren't many companies that have a proven track record in this new field.
4. Self-publish and do it all yourself. The advantage is that it's cheap and not beyond the abilities of most writers (you may still want to have an editor go through the book and get a graphic designer to do your cover--bad covers kill sales and are common among self-published ebooks).
THE UNCOMFORTABLE TRUTH
Here's a hard fact: publishing an ebook is the easy part. Selling it is the hard part.
Yours will be one of tens of thousands of ebooks out that week.
Sure, you can get it listed on Amazon, the Barnes and Noble site, and the rest, but what's going to make people look for your book?
How is it going to stand out?
Are you prepared to spend a lot of time using social media to spread the word?
I'm not trying to dissuade you from publishing your ebook, and i hope the summary above helps you decide which way you want to do it. But do be prepared for the fact that publishing the book is just the first step!
(Working on a book? Get friendly guidance from idea through to publication from my book, "Your Writing Coach," published by Nicholas Brealey and available from Amazon or your other favorite bookseller.)