There are a few considerations when deciding whether you want to purchase your own ISBNs or not. First, if you want to save money and only plan on selling your books through CreateSpace and Kindle, then there is no reason not to just use the CreateSpace ISBNs other than for vanity reasons.
Having your own ISBN number is useful if you want to have your publishing company listed in Amazon. You can always list your own publishing company on the title page and copyright page of your book, but if you want libraries or for the listing on your Amazon sales page to have your publishing company then you will need to have your own ISBN. For most people, using the create space ISBN number will not be very visible and will serve their purposes fine without having to spend any money.
The cost of an ISBN very is widely depending upon what country you live in. If you live in Canada, for example, then you can get your ISBN numbers for free because registration is handled by the government. If you don't mind spending a little time registering them, then you may as well take advantage of that program. In the United States, ISBN numbers cost between $125 for one and $1 each for 1000 ISBN numbers. In Australia, it costs $42 for one ISBN or $2.82 each if you buy 1000 ISBN numbers. You can determine where you purchase your ISBN numbers by choosing your country at the International ISBN Agency's website.
If you ever plan on selling your books anywhere other than on Amazon, then you will not be able to use your CreateSpace ISBN number. The platform that you use may or may not provide its own free ISBN number, but if your book is one addition then it makes sense to have the same ISBN number for all places that it is being printed. In that case, you'll probably want to purchase your own ISBN numbers.
If you are publishing multiple editions of your book, you will need a different ISBN number for each addition. For example, a hardcover and softcover book will each need their own ISBN number, as well as the audible version of the book and the e-book version. If you are only selling your e-book version through Kindle, then you will not need an ISBN number at all as Amazon does not require it.
There is definitely value in having your own ISBN numbers for branding your publishing company. However, when you are starting out it is not something that is a very high priority. I would recommend holding off purchasing your ISBN numbers until you have a lot of business and it makes more sense for your publishing company. You may find that you don't need to purchase them at all; John and Jay have not purchased their own ISBN numbers yet and they have hundreds of books for sale on the Amazon marketplace.