I'm a self-published author, I write non-fiction. I would like my book to have a reasonably good cover and stand out among others, but I'm not ready to spend more than $500 on it. What are my options?
I think there a few different paths you may go here.
One way would be to try to find an illustrator who'd sell you the work of his own to be used as book cover. You can do this on sites like IllustrationMundo, DirectoryOfIllustration (beware, lots of agents instead of actual designers on this website) and HireAnIllustrator. It may be pretty difficult to find relevant works there, because those sites allow to search by illustrators, but not keywords that might be relevant to your book.
Another way is to try to craft one yourself with numerous software available. Just type in "make book cover" in Google and you'll be enlightened with tons of cheap-looking software packages promising you super skills in designing a book cover.
Finally, there are stock images websites where you can purchase an image to be used as a book cover. The most notable and closest to the book industry is Get Book Cover, which sells fine-quality illustration and photography works as book covers, allows keywords search and claims to have an in-browser editor to put title and author credits on the cover, as well as convert it to various formats for digital and print use.
$500 is more than enough to get a pro to do your cover for you. It wouldn't be enough for an individual photo shoot, I don't think, but there's lots of people make stock-photograph covers for a couple hundred bucks.
Find a self-pubbed book with a cover you like, and see if you can find the cover credits. Or, hell, find an industry-pubbed book with a cover you like; a lot of their cover artists are probably working freelance.
$500 is enough to find a good illustrator / designer. Look for someone who is starting out and wants to build a resume (not someone who's already established). With some work, you can definitely find a good young illustrator at that price range, and you'll help them in the process.
You might want to look at some of the artists listed here in the KindleBoards Yellow Pages. You should be able to find a very talented artist who can help you develop an e-book cover for under $500 easily. Some of the folks listed here are mediocre, but there are many who are amazing! I personally have used Glendon Haddix for two of my book covers and have been most pleased with his work. He does excellent work and is very responsive. (He is also well within your price range.)
Some of these artists provide premade covers at very low prices, and most of them will provide examples of their work. Take the time to look at a few and see if you find someone who has done something that catches your eye. I'm sure you won't be disappointed!
Find a good artist and offer them a reasonable contract, but whatever you do, don't promote spec work! As a writer you should know better. Contests produce some good results, but that kind of thing devalues all of our work as artists.
You could try holding a design contest on 99Designs. I just ran a logo design contest for $500 and received 50 different designs I could choose from and was very satisfied with the results.