A couple would be: AP images (pricey) istockphoto.com Getty Images Images will cost $1 to $100s each depending on usage, quality, source, etc.
There is no average price. It all depends on the quality, content, and demand. You can search for "public domain images". Those can be used for free. But be very careful. Many of the sites that claim public domain images really just have lists of images they took from other places. Also, this forum is about copywriting (see the 'w') which is the art of creating ad copy, not copyrighting.
I usually check www.sxc.hu first. Most of the photographers there allow you to use their images for free with credit. If I can't find what I want there, I buy them from www.StockXpert.com. I used to use www.iStockPhoto.com, but I found the other site tended to have better images for my needs. You can get them for $1 to slightly over that for small images (large enough for blog posts), or pay more for higher resolution ones). If I need product images or something, I look for a press section on the company site, where they make the images available for reviews, news posts, etc.