If you are going to independently publish, you can’t do it for less than $150 (I found one place, but you have to print with them in order to get it for $99, which, in the end, didn’t end up being worth it).
Your ISBN, International Standard Book Number, is like the Social Security Number for your book. Every book with shelf aspirations needs one, and each different version (i.e. an E-book) should have one. Book publishing companies buy them in bulk and get them for about $27 a piece. So, should you go with one of those outfits and they want you to pay more than $27, tell them “Sorry, Book Social Security Numbers don’t cost that much.”
If you blaze your own trail and can buy a block of 10, that will cost you $270 + $25 per barcode, which is economical in the long run. For one book, it costs $125, plus $25 per barcode to make your dream come true. $150 to get started. That’s all it takes.