How to price your drawings:
1) Pick a $/hr amount that your time is worth. Make this above minimum wage.
2) Track how long it takes you to draw pictures. Manictime is a great time tracking program for digital folks.
3) Multiply time spent by $/hr.

4) Feel like you're overpricing, wallow in low self-esteem, wheedle to yourself that nobody's going to bother paying you, and have a mild existential crisis for a week or so.

No matter how skilled you get, that little voice will always be there second-guessing your AUDACITY for DARING to charge SO MUCH, how DARE. You are worth more than you think. If you treat your art like it like it is valuable, so will other people... and vice versa.

But once you're done feeling insecure, either chin up and price yourself at least minimum wage or DON'T TAKE COMMISSIONS until you can justify asking minimum wage. If you undercut yourself, you don't just hurt yourself, you hurt every other artist trying to sell their work.

This gets mushy when dealing with virtual money or like, very clearly novice work where you just want a few bucks... but what I usually see is if someone's at the point where they would even consider doing Real Commissions, they're usually also good enough to charge min wage.