How much do you think I could charge for art like this?

There's a sticky near the top that says do not ask for pricing here because there is a subforum for it in the art discussion area.

It's pretty good but the fact that you shaded the moon with a greyish colour makes it look a little dull, try shading with other colours . more saturated maybe?

I like it but your lines are a bit wobbly and the shading looks a bit dirty. Did you maybe use the dodge tool? If so I would suggest refraining from doing that next time because it tends to make shading look dirty and uneven. Other then that it is a great drawing. No clue on prices though, not good with that sort of stuff. But like Ryalee said there is a subform for that. ;)