For ebook covers, my base rate pricing ranges from US$90-$150, for up to 3 stock images. Fewer images means lower price, basically.
Prices can be negotiated for any work requiring more than 3 stock images.
I use a cover sheet to establish a strong idea of what an author is seeking before I set "pen to paper". This enables me to hit pretty close to the mark with the first proof in most cases.
I also offer a more budget-conscious version. If an author supplies high-resolution images for me to work with, the price drops by 50%. So the price then becomes US$45-$75 for a cover.
Why the reduction? Because of the saving of both time and money. Searching for images is a time-consuming process, and purchasing them is obviously an expense. If those steps are taken away, the job becomes much quicker and easier. Another bonus for the author is that they then hold the licence on those images, rather than me holding it.
Please feel free to contact me at: firstname.lastname@example.org