Youre planning on creating a One Shot, might as well give the new 2020 contest a try. Well, only if you ever find someone to do the art and make it in time.
For your question. I guess around 8-10 $ per panel is a good place to start when the artist is at entry level.
12-15$ per panel when the artist is experienced and precise.
And more than 30$ for high level input I guess. I dont have an estimate on this high level of professionalism so I am just nitpicking but the entry level and experienced level, that I could vouch for.