This is a nice article; when I first did a commission, I wasn't sure how to price it either, but my dad was actually able to help and he said the same thing about getting paid by the hour and that you want it to be worth it. Your artwork is worth the 'normal hourly wage' price, and if someone's commissioning you, obviously they think so too.
For me, it's not the pricing now that I'm uncertain about, it's getting customers.