Someone I know posted a link to a very interesting blog post recently regarding the costs involved with self-publishing a book. The costs listed were those needed for professional editing and cover design services.
Personally, I find the numbers quoted in the post a bit too high. I can see them as certainly being prohibitive for the average person trying to self-publish.
This doesn't mean that I think authors should try to do it all themselves, however. A good editor and a good cover designer are worth their weight in gold. So this is where you start doing some research. Ask around for referrals. Your writers' group should have some information (and if you aren't a part of one, you might want to consider the idea. One or two serious critique partners are also worth their weight in gold) or you can utilize your friend and mine, Google.
If you're attempting to make your writing into a career, you can push the odds of success farther into your favor by making use of the resources available to you and doing what you can to produce a better-quality book. With a little effort, you should be able to find reputable editors and cover designers whose rates fall into your acceptable range. They're out there, so don't stop looking! Check for their work history and experience, and if there's a list of testimonials or former clients, take a look at that, too. It shouldn't take you long to find someone who suits you.