It's $125 list price for a single ISBN, but there are bulk discounts buying direct and purchasing through a large scale supplier can make them as cheap as $10 each. There are deals to be had; Amazon, for example, may give you a free ISBN for your ebook as long as you publish it using KDP, their walled-garden publishing system, but the gotcha is the ISBN is not portable/you're not permitted to use it for other editions outside of the Amazon system.
The other downside to these free (just about always) and discounted (sometimes) ISBNs is that they link the publisher as the service you got the ISBN through, rather than yourself, even if you're doing what would classically be considered a self-publishing job. How big of an issue is that? IANAExpert, but it seems like there are some nooks and crannies of IP law that can be swayed by owning the imprint, but little practical concern for the average person putting an ebook on Amazon e.g. Perhaps someone with more in depth publishing knowledge can color the risks better than I.