The answer is that there is no definitive answer. The cost to self-publish a book depends on a large number of variables, including but not limited to: The size and complexity of the book, the number of illustrations, the number of specialist services you require, whether you are publishing for print only, or for an eBook only, or both, the number of illustrations in the book, the number of books you would like printed, and more. You can get an idea of how much you book will cost bys using Cost Estimator. 

Total control – design, content, cover.
Total Ownership and Retention of your rights.
You can publish quickly.
You can publish in paperback, hardback, and as an eBook or audiobook.
Print-On-Demand means financial risk is reduced
You have numerous non-traditional distribution options, especially online.
You can target your niche market.
You earn royalties on every sale.
You are guaranteed to see your work in print!
Your book will never go out of print!

Even the best things in life sometimes have a downside:
You have to do your own marketing and distribution
You have be an entrepreneur and business person
Very few “Indie” authors make it big

No, I do not. I simply lack the resources to do that, and to a large extent, you are on your own. However, even in traditional publishing, a large part of the onus to market the book rests with the author,
However, here are some ideas you could use:

Traditionally, self-publishing authors would have to order large numbers of books to be able to get an affordable "per copy" cost - this almost inevitably led to stockpiles of books in the author's garage, and a huge capital outlay! However,  thousands of books are now being self-published annually all over the world, at far lower cost to the authors.
What caused the change?
The development of Digital Printing technology, as opposed to the Lithographic process, has been one of the contributing factors.
We all know laser printers and a commercial Digital Printer is simply a very large laser or inkjet printer.
This allows for:
Fast, quality short-run printing
Economical pricing for short printing runs
No expensive book inventory
Print-On-Demand - digital printing means you can produce one book “on demand” - ie when the reader orders it. This has become the basis of how Amazon and other platforms sell books online - Amazon has set up "Print-On-Demand" facilities at numerous locations around the world. 
Another benefit of Digital Printing and Print-On-Demand is that you can make corrections and changes to your book between print runs at minimal extra cost.

No, I do NOT! You retain complete ownership and control of your book at all times. Look at me as if I was the builder you hired to build your house - once he is done, the house is yours to do with what you will. Once I have "built" your book, you are free to take any action you wish. For example, if it starts doing well, you may want to take it to a traditional publisher. You don't even have to ask or tell me!

ISBN stands for International Standards Books Numbers, and is an internationally recognised identification number for books, which is mandatory for commercially available books. It is a publication’s fingerprint, a world-wide identifier that helps publishers, suppliers, and libraries locate, order, and track published material. It is also used as the basis for the book’s barcode, which is mandatory if the book is going to sold in a retail environment.
You will need a separate ISBN for every version of your book: Print book - softcover, print book - hardcover, eBook and audibook.

There are numerous online platforms to which you can upload your book. The best-known ones are KDP/Amazon, IngramSpark, Barnes & Noble, Draft2Digital, BookBaby, and many more. Some of these also make your book available in physical bookstores. Each of these online platforms has its own criteria for accepting books and paying royalties, and some charge an upload fee. I will assist you where I can.

We prefer all manuscripts to be submitted in a Microsoft Word document. However, we will accept a wide range of other document types. PDFs, RTFs, ODTs or InDesign files. However, some of these file types may cost an additional fee due to the added work needed to convert them.

Of course! Your files are your own property! I provide an “author’s copies” print facility as a service to authors, based on a quote, but you are not required to use this service. Just let me know what your printer’s specific requirements are before ordering, and I will be able to deliver a print-ready file to you.

I understand that often, there are going to be corrections that authors wish to make to their manuscript once they see it in "book" format. I include one free formatting/layout/design revision with my formatting package. However, you will only receive one free set of edits for changes to text, headings, errors or typos unrelated to the formatting and layout process. After this, edits or “author’s corrections”, will be charged for.

Yes, of course! All graphics should be Jpegs or PNGs, ideally at a minimum of 300 dpi. However, I pride myself on my ability to manipulate and improve pictures, especially very old photos digitally, so do not be too concerned about the quality of the pictures you provide. It is important to let me know where each image must be placed in the manuscript and to provide captions. There is a graphics handling fee based on the number of graphics supplied.