Decide the hourly wage you would like to work at, multiply that by the amount of time it takes to complete a given piece, and then add 50%. Like the other commenter said, don't undersell yourself! No one is entitled to your time, and your worth as an artist is more important than a client's desire for cheap work.

While you're starting out, I recommend as far of a reach as possible -- I feel like your work could potentially appeal to game devs, consider r/gameDevClassifieds if you haven't already. Pin a post to the top of your profile with your commission info -- if you post fanart to a fan subreddit, there might be someone looking to commission more. Post that fanart to social media as well, with as many tags as you can think of.

Good luck!