How much time does it take you to do X? What's a good hourly wage?

That's one way to approach it.

So I will try to take some commissions this evening to try and get back into it. Hopefully folks will have a good time. :)

Start with prices based on something like $15/hr plus expenses. (i.e. Don't rip yourself off.)

After a couple of months, start adjusting that price to get things to where the number of slots you offer and the number of people who want to take those slots are within about 10% of each other. (i.e. Don't be the guy where there are more people bemoaning missing a slot than you are actually getting to do art for.)

Be up front that your initial price is temporary and that it's probably going to change a couple times this year.