I'm not really sure about prices, but I wanna say this. x3
You could probably benefit quite a bit by working on your presentation. What do you see in every single one of those pictures on your dA? Shadows. Shadows everywhere. It really won't do your drawings justice if you present them like that.
Especially the one on the notebook cover. I can tell that one is very nice.. but it doesn't look quite as nice as it could because of the uneven lighting. Makes it look off somehow. And that one doesn't even have shadows as bad as most of the others.
When you take your pictures, try finding a really well-lit area, where the whole image will be completely lit up. Maybe outside or something. And make sure you're not standing in the way of the light source, like it seems you did for several of those examples. It could also help to take the image into a digital program afterward and lighten things up. I usually do that by alternately sharpening the picture and reducing noise, until the background is as white as possible. But of course, I'm sure there are other ways to do it - and probably better ways. *shrug*
Oh, and there's also the fact that you tend to use lined paper. That's another big quality reducer, and really one of the easiest to avoid. You should always try to do your art on blank paper. Even if it's just simple computer paper, it tends to look much cleaner and much nicer without the lines cutting through your image.
... Bah, ramble. xD
Well. Good luck on your commissioning and stuff. ;o