"Fair" is subjective. One artist will think themselves underpaid for what another will consider good money. I think it depends on the difficulty to meet requirements, experience, speed, the standard of living at both customer's & artist's locale, & more. So, I think your answer may only be known when you've made a deal with a particular artist.
I also think artists maybe think about "the min wage" too much. I mean, min wage is only a legal construct. All it does is say how little a legally-defined employer can pay a legally-defined employee. But an artist is their own employer & employee.
Also, min wage changes arbitrarily; one day it's (e.g.) $7/hr, another day (by law) it may become twice that - but the art & the artist didn't suddenly get twice as good. So, the legal construct & it's arbitrary number don't necessarily apply unless you make a personal decision to make it so.
A further thought: Min wage is the price of labor. It is how you buy the time of an employee (presumably also the effort but only if the employee is diligent). Most people recognize how paying a worker by the hour can make a job more expensive than if they'd agreed on a price for the job. And buying art is almost always an exchange of money for a product, e.g., in terms of frames or pages, instead of giving money for effort. I can work really hard for a really long time & still not deliver anything you'd be happy with! 
