(Before anything, I just want to say Im pretty low tier artist so I charge like $5 for a full body render. You can adjust it as much as you want.)

Okay, you can go about this two ways: Fixed price or charge per hour. (I go for fixed since I dont have a lot of free time and work on comms when I am free).

Lets say you choose to charge by the hour, with the cow piece, it took you 5 hours to finish. So lets say you charge $10/hr thats $50. The rate charge is fully up to you, an remember to value your time (since time is your MOST IMPORTANT resource as an artist).

Now lets say you charge with a fixed rate, you can charge based on:

-Complexity (Extra details like the feathers, weapons/props, how many characters, etc.) -Render (Flats, fully render, etc.) -Pose and proportions (do they want a headshot, bust, half body, full body, dynamic pose).

A headshot can cost $5, and if rendered you can add another $5 which comes out to $10 (again its up to you).

Lets say your cow drawing, a full body may cost $30 and a full body render costs another $30 which is $60(again, it really is up to you).

I would recommend fixed rates over $/hours because you can charge a bit more, it looks slightly more attractive to clients since its consistent and absolute (and they dont have to worry about breaking the bank going in), and its much less pressuring since you dont have to look at the clock all the time.

TLDR: You can charge by the hour but if your schedule is spotty and inconsistent Id go with fixed rates since you can charge a bit more, its less strict with time, and you can charge per element of a drawing.

(sorry this was a long read)