> average minimum wage per hour While this is good as a baseline for weeding out scammers, your artwork deserves more than minimum wage! Charge what you believe your art is worth. It's common practice to add extra cost considering that you're licensing your work for commercial use.
I haven't done exactly that, I tried and only got disrespectful scam offers so I can beware you of that. You will have adults making bank off of your work so do NOT accept only $5-15 per skin. Try for an average minimum wage per hour you spend. Some teams also offer a % from what sells.
I have done work for a company who publishes skins on the Marketplace, but not as a solo creator. I don't know if that applies to your situation, but hopefully I can help. I also know DragonsDungeon has done Marketplace skins.