Also, what rights do you usually get w.r.t. said works? DarkGeometry doesn't list a standard contract on its site, but the rights I'm most interested in are:

1. The right to use it as a cover for my epub/mobi book and sell said book on any digital market I please without any royalties to the artist (onetime payment only).
2. The right to use this as a cover for a physical book, should I decide to make print-on-demand copies.
3. Access to editable versions of the files, at least as regards placement of the title of the book or my name. For instance, if I should want to publish on market X, but X requires the author's name to appear in N-point Helvetica font at the bottom of the cover, then I want to be able to do that without being at the mercy of someone else's schedule or touch-up pricing (no matter how bad that may ruin what the artist was going for).
4. The right to use the purchased cover image at any resolution.
5. The right to use said image anywhere I want to represent my work on the web (provided I don't try to claim I made the cover), be that on my own web site, on the site of someone doing a review, giving me a recommendation, or basically anywhere I consent to have my epublication mentioned.
   

Are most of these standard? Do you pay more when you want any one of them? Are certain artists that you guys have worked with more clingy about them than others?

She's running a $50/cover special right now, which is very reasonable and mid-low for price ranges for the people I've seen in the cover space.

There's more involved in doing a cover for POD book, because you need not just front 5x8 cover art, but also spine and back. So be prepared for cover designers to quote you two prices - one for just a flat cover, something higher for the whole wrapper.

Very few artists I know will deliver you original work for $40 a pop, but most will happily make small changes for low $, or give you a few versions for the set price you negotiate up front.

One of the whole points of epublishing your own stories is so you don't HAVE to bend to the details of specific market requests - you put your cover on your book and self publish it on whatever platforms you decide - smashwords, barnes&noble, amazon being the top 3 (smashwords gets you into the Apple iBookstore, as well as onto the Sony Reader and Kobo platforms, which have large installed bases and international readers who are hungry for english-language content.) They don't specify anything about cover other than max filesize and certain dimensions (B&N needs one slightly longer as I recall.)