I'm married to an artist who finds herself grappling with this very question sometimes, so I've had many discussions about the subject. Please allow me to share my viewpoint, as someone who's more on the consumer side but is close to the creator side as well.
A couple of factors I think go unconsidered sometimes is A: Notoriety, and B: Time.
Notoriety
Simply put, your prices grow with your following. Starting off you might charge a bit less than other, more well known artists who put out comparable works, as this attracts commissioners and builds your base. On the inverse side, however, there comes a point in a successful artist's career where their time and effort comes at a premium due to scarcity. If an artist is always booked solid at price points they feel are a bit on the low end, that's a good sign that they should increase their rates and emphasize quality over quantity, both for the sake of making more off of their efforts and so as to not overwork themselves and end up burning out. You don't want to undersell yourself and get buried under too many commisions/requests, but you also don't want to scare customers off by way of sticker shock.
Time
I'd argue that time is the biggest expense/investment an artist puts into their work, more than supplies or equipment.
Try this as an exercise:
1. Write down a list of types of work you want to do for commission We'll call this Column A
2. Next to each work, write a rough ballpark figure for how much you think you should charge for that work. - This will be Column B
3. Add a third column (C), filling in the number of hours, on average, you will need to invest in creating a satisfactory work of each type.
4. Add a 4th column (D), and fill in B/C to get an hourly rate
Now, ask yourself "Are these rates consistent?", and "Are these rates both fair to me, and attractive to customers?" Most peoples' frame of reference is pegged on an hourly wage from past and/or current work experience, so it's certainly worth considering this angle.
You don't want to charge $10 for something that takes you an hour, but $15 for something that takes you three hours. In that circumstance, you're either overcharging for the $10 tier or undercharging for the $15 tier, relatively speaking.
You also don't want to find yourself making $5 an hour because you feel guilty charging $50 for a 4 hour project, which is right at or under minimum wage in a lot of places. It's easy to focus on the end result of your work and lose sight of what you're investing to create that result.
All of that aside, you can also take a look at other artists making similar artwork, and see what their published prices look like. Such a baseline can help oodles, as it gives you a starting point to go up or down from based on following, quality on smaller touches, etc.
This reply ended up being a lot more verbose than I intended, but I hope it helps!
