Wow, good work! What size? Resolution? Level of detail? Do you want the artist to also do the layout? Will it be a wrap around cover, or just a front cover?

If it's a wrap around then you might be interested in this artorder challenge:
http://artorder.blogspot.com/2009/08...ge-part-2.html

As for ballpark prices, it's going to be a couple of hundred dollars at least I'd expect, but I'm not an expert. You can certainly contact a few artists and ask for their rates for the task you are describing - and the more precise you can be, the more accurate their quotes will be. However it's certainly better to have a figure in mind before you go asking. You'll find a lot of quotes will be well above the couple of hundred dollar mark. The trick is in finding someone who wants the experience and is good. ConceptArt is probably a good bet for that as there are lots of art students on there.