Listen, I am not a visual artist myself so I cannot speak with certainty, however your prices are in accordance with what I have seen other visual artists ask for their services.
I believe however that in some cases (and again, don't take my word for it), some artists will do the digital coloring for prices that may range from 30 to 50$. But this really depends on the artist and as well on the demand for their services. Artists that are very in demand can permit themselves to charge more (I mean, basic economics demand versus supply). However, at the same time, I am pretty sure I've seen most artists ask at least 15$ for sketches.
As said above, I believe everything is quite in accordance with what is generally asked.
I wish you all the luck with your art and your commissions! I hope you make plenty of money.
And you are a master colorist, let me tell you. Really beautiful art!