Prices range from a laughable 14 cents to about a hundred bucks, depending on size, resolution, and what the photographer feels like charging. I paid $60 for one cover shot, but most of my pix cost $4 or $6 each.
Unlike some "stock photo house" policies, you are buying a license for nearly unlimited use. You don't pay more money based on the readership/viewership of your media, or the purpose of your project. All the files available on Fotolia are royalty-free, meaning they can be used with no limits on time, number of copies, or geographical location.
About the only limitations are that you can't put any person in a photo in a bad light or in porn or a violent situation, or use a photo to support a political party or religious organization.
One other possible limitation is that despite a nearly endless selection from Fotolia and its competitors, you may not find a photo that's exactly right for you.
That's where someone skilled with Adobe Photoshop can remake a stock photo into a custom photo.
The photo in the book cover at the top was nearly perfect, except for a generation gap. I needed a picture of a father speaking to a child, but the original man (in inset on the right) was obviously old enough to be a grandfather, or even a great grandfather.
Carina, my ace cover artist, gave him a hair transplant, eliminating the effect of decades.
In the seond row, Carina doctored my 1971 wedding picture, to remove my wife, clone some hair onto the right side of my head, and remove a reddish cast from the photo.
In the bottom photo, Carina removed a cluttered background, straightened out my tilted head, and removed my right hand that looked like the deformed appendage of a Thalidomide baby. In the future I may ask her to give me more hair and darker hair, remove the wrinkles and change my eyeglass style.
Carina is too busy to work for others right now so I won't tell you how to find her, but you should be able to find a suitable substitute.