Oh goodness, I think you are undercharging by a lot!
For all intents and purposes, let's just say one apple is worth $.50 USD. I got this number because 10 apples is about $5 USD.
So right now, for those full gorgeous scenes which I bet take a lot of time, you are only charging $5.50.
How much do you want to make an hour? I usually say shoot for at least $15 an hour. That is 30 apples an hour. Based on this, figure out about how long each piece takes you, add in any extras for complexity and mental fatigue, and there is your price. Many people will undercharge while they are new to selling to build a reputation, which isn't a bad idea.
If your headshot takes lets say one hour, I would not charge any less than 15-20apples bare minimum. I personally would say 30-45apples for a headshot is a fair price for your work, you have so much detail and care into every inch of your pieces.
Seriousness aside... OMNOM EATS YOUR ART SCRUMPTIOUS ART DELICIOUS !!!