Depends on how much labor and time went into making the commission IMO
Its kind of a self-worth question though. If you don't think you're worth more than $30, you won't get more than $30, even though you should be getting more. You are ripping yourself off in a sense
How many hours do you usually spend on one? How much effort and concentration per hour? maybe you ate a few things while doing it. How much did the food cost?
If the commission earns enough money to buy back the things you used to make the commission (electricity, food, internet, etc), then thats good, but you could charge more and profit from it (I think thats where people liking your style/you being famous comes in) because then your pictures have added special value
again this is just how I look at it: if you don't think you are worth anything, why should others think you are? *shrug*
I agree with whoever said you are undercharging. A full color from you is at least $45, you make amazing armor and accessories