Click to see the actual size of the banner which is 1000 x 200 and 77.1KB
Karina commissioned a custom banner, with various elements from her writing. My custom banners start with either a stock photo or digital background and I add illustrated elements. Hers are a spaceship, dragon, zombie and catholic item (which ended up being a rosary). I thought it would be interesting to have all these things floating in space, since her writing is sci-fi and fantasy. I referenced covers of her books for most of these.
The main title font in her banner is one I downloaded and customized for her. I'm careful to use fonts and stock photos that are royalty-free. I work mainly digitally in Photoschop CS5 on a Wacom tablet, but I can also work traditionally with pen and ink, colored pencils, and watercolor.
My custom banners range in price from $70-$90, depending how much detail and time is put into them. Karina's was $70. A banner with a finely-detailed character portrait, or a digitally-painted landscape would run $80-$90. These can be any size, but I recommend larger ones like Karina's so that I can get more detail in, and you can get more for your money.
If you're interested in a custom banner from me, send an email to firstname.lastname@example.org with "banner commission" in the subject. I believe in lots of communication, and working closely with clients to understand their vision and produce something that will blow them away. (I also have a lot of fun in the process.)