I recently started selling at artist alleys! C: it took a bit of trial and error but I finally figured out how to price my stuff.
For prints, I have around 4-5 different sizes.
4x6 = $1 each (super cheap to print and people really like them, I've noticed!)
5x7 = $2 each or 3 for $5
6x9 = $4 each
8.5x11 = $5 each, 3 for $12
11x17 = $13 each, 2 for $20
I price keychains (the laminated ones) $2 each, 3 for $5, 5 for $7.50 so on and so on. I price pinback buttons the same if they have an illustration, instead of text. :> As for polymer clay/acrylic keychains, it depends. I know some people sell polymer clay keychains depending on how long it took to make them. I only make little sushi keychains and I price those at $4 each because they're easy to make! It depends, really!
I've never done on the spot commissions but a friend of mine charges based on the following:bust only, waist up, full body, whether it's pencil only, inked, coloured in, etc. I'd go with what you feel is more comfortable for you.
I hope this helps!