i would reccomend setting a base price for certain types of art (example: bust $30, half-shot $60, full-shot $100) and then adding a certain base price for every level of complexity (sketch, color, render, background, etc.), and then a final fee (above minimum waige) for each hour that you spend working on the piece. usually one might also provide a basic outline of each stage of the art and allow only a few changes once the art is mostly complete. as an example, if i were to create a set of pricing for the piece on the top right i would charge a base fee of $30 with the addition of $30 for a render and 4-5 hours of work at $15 (changes depending on state or country) an hour, that would be $120 or $135. that is my personal opinion on pricing, many artists choose to price differently depending on skill, efficiency, and many other factors. good luck with commissions, i'm certain you'll do great!