When you write a book, you often dream up what the cover will look like. We all do it. And if you're lucky enough to get published by a traditional publisher, you'll likely get a really terrific cover done by professional and trained designers who know how to create an eye-catching book cover.
When you self-publish, this task is up to you. You have control over all aspects of your book, and this is one of the reasons many people like self-publishing - freedom to do as they have envisioned for their book.
But this doesn't mean you should make your book cover in Pages or have your Uncle Frank do it for you because he has a computer and time. You have to pay a professional. There are many freelance graphic designers that can give you a great book cover, a cover that won't scream "I'm a self-published book!"
Death to a self-published book is a poorly rendered cover (and of course a crappy story that wasn't edited to boot). They often look cheap, and have no design sense to them. Pay for a good cover. Get a professional and look professional.
You've come this far - why cheap out now?