Depends on your art style like others have said.
What I would do is go through art sites like Deviantart, art station, even using the #commission hashtag on Instagram and Twitter to see what people are charging. Find like 5 or so commission pricing pages that are similar to your style and talent level. I highly suggest taking maybe 2 from each platform because different platforms have different pricing depending on who's on them.
After that I would organize your prices something similar to this:
Price for:
Sketches
1 character bust
1 half body character
1 full body character
Colored Sketch
1 character bust
1 half body character
1 full body character
Lineart
1 character bust
1 half body character
1 full body character
Colored Lineart
1 character bust
1 half body character
1 full body character
You can add additional prices like +$$$ for each additional character and +$$$ for NSFW
Hope that helps! That's just what I've seen in my experience! Also always get at least 50% of your payment up front when accepting a job.