No real clue on what you should be charging for your work, but looking through your gallery I'd say you certainly possess skill in rendering images, but with you asking about taking on commissions my question then becomes how much time does each category of rendering take for you. Once you know that, it should make things easier to with a starting rate a few dollars above the hourly minimum wage in your country. You use this time to develop your preferences when it comes to this whole thing and then raise rates as you increase the time you invest into taking on client work.

Useful stuffs in a price sheet imo:

- icon (MostlylI saw prices between $10-30)

- chibi (if you draw) ($15-40)

This depends on your style:
- sketch (fullbody) ($10-30)
.
.
.
- colored & shaded (fullbody) ($30-80)

- simple background ($10-30)
- detailed background ($30-100)

- reference sheet ($30-150, depends on complexity)

- badges ($30-80)

(- additional prices (wings, tattoo, etc.) )

This price sheet is really depends on your style/what kind of stuffs you draw. You can just put one or two example for that monochromatic painting and the price if it still fits on your sheet.

These prices are just suggestions. I don't exactly know your need, how many hours you work on a pic, etc.

Goldenavatar has a good point. It really depends on how long it takes you to draw a picture, obviously the simpler the picture the less time it takes and the cheaper you can offer it. your $35 busts are very well priced, you could probably gauge off of that.

I also agree with time-spent prices, also remember that as artists we tend to price ourselves lower than we should, don't forget to include the time you spent doing sketches and preplanning as part of the hours to calculate the fee, a lot of times doing a finished piece of artwork takes us longer than we really think!

And good luck with your commissions!