I charge depending on complexity from a base price. For example, something will start at $5 and then work itself up to a final price depending on the details you want in your commission.
Twitter banners: starting at $10.00 Youtube: Starting at $12
Other: Starting at $7.00
Avatars/Icons: $2.00
Twitch Buttons Starting at $1.00 each
Stream Layout: Starting at $13.00 Offline/Placeholder: Start at $10.00 Both: Start at $16.00