With my own characters these only take an hour, since I'm familiar with drawing them. When drawing someone else's character it takes a bit longer, especially since I tend to make them more complex (closer to the Fairy Ring example). That one took about three hours. So, being conservative, if I'm working 2 hours on something for $15, I'm working for 25 cents more than minimum wage, which I think is fair.
It's a lot easier to find commissioners if you spread out. Tumblr, DA, FA, Derpibooru. Make sure you go by one name all around, so that people can find you wherever they see your art. Also have a logo or put a small URL in the corner of your pictures. Your art is really good! You just need more exposure, I think. Also try experimenting with different styles, do a bunch of canon characters (those get a lot more attention)... that sort of stuff. Haha, that's what I did to get some exposure on tumblr. I also do streams and requests occasionally. Hope this helps c: