Well... First of all, I don't think that many artists will accept Ink as payment. They rather have actual money, you know, to pay for their roof, food or other expenses. Not to mention that they don't have to wait for the money to first be available within the app, to later extract it to their virtual-wallet (Paypal or similar).
It'll be a matter of searching, since artists's prices vary from where they live, their experience, currency exchange, prestige, abilities and more. It'll all depend on the type of artist.
You'll find some here if you were to open a "Looking for an artist to draw my Cover" on the Collaboration category (Make sure to clarify if you're willing to pay, an approximate budget if so and such, not necessarily info about the commission itself, but to give an approximate idea about what you want), you can also search for artists through Twitter, Instagram, Facebook and other social platforms, still, be careful of tracers and scammers.
When it comes to prices, you're going to find almost... anything, but cheap doesn't always mean good (In the sense of service or ability quality or even delivery time) so always be careful. You're going to find talented people charging less than what they should, or the other way around, prices could go from $5 to $100 (Yes I know is such a gap, but that's how the market is), and prices may vary depending on the amount of characters, if there is a background, commercial rights, canvas size, and similar.
