I researched cover art extensively several months ago. The difference in the pricing is that there seem to be two different types of cover artists out there. Those who are artists who create original art (digital or otherwise) tend to be on the higher side. I can think of one artist who I'd love to have design my covers (one of our fellow AW members). He's a bit pricey though since he does fantastic original art and my budget right now has to focus on the production of the audio books.
The lower cost covers tend to be by graphic designers who take stock art and manipulate it in one way or another. Some of the designers seem a bit more limited in what they can achieve (as in, if there isn't stock photography that can be used as the basis, they are less likely able to create the desired cover). I had one lady explain to me very carefully why she could not produce the cover I had envisioned. Apparently, the idea of taking two or three stock images and manipulating them to create an entire new image was not something she was interested in doing. Her alternative suggestion, while charming, was more suited to an historical romance (and similar to other covers she had done) and had little to do with the actual story or what I wanted.
Many of these covers by graphic designers are stunning, however, it explains the price differential. It also means that a writer must be realistic in his or her expectations of what a particular cover artist can generate.