I take the wage that I'm okay with and multiply it by the average time it takes me to complete the request. Another great way to gauge pricing is to look at how other comparable artists price their work.
So let's say I'm willing to work for no less than $25 per hour if I like the requested subject matter, and I think the piece will take me 5 hours, then I'll charge $125.
Here is another factor: how much do I like or dislike the requested content? If I politely want to refuse work, then I just charge a higher rate like $50 per hour, hopefully high enough to make them not want to say yes.
Here's a link to wages from the animation guild. These are studio rates, and freelance rates are a little higher but if you're new then I wouldn't go too much higher or at all if it's hard to land real gigs.
http://animationguild.org/contracts-wages/
Another note is that comic book artists generally make a little less than animation and film artists. That is another factor to consider.
Two facebook groups also are great for informing on commission and art prices for comics:
sketch prices - comic book sketches, commissions & autograph prices
This group tends to post art from higher end and popular artists in the industry so prices are usually a bit higher than average.
Connecting Comic Book Writers and Artists
This group usually lists mid to low level artists, and sometimes are priced too low in my opinion but they also include outsourcing from other countries.
Hope this helps.
