I know Ethan sometimes does map commissions (dont quote me on this), so maybe reach out to him in private and negotiate something?
I doubt any mapmaker will just run into the forum and tell you their hourly rate.
But according to this https://www.bls.gov/oes/current/oes271013.htm
an US based artist makes about $30 per hour, and map making takes a lot of time, so dont expect this to come cheap. Obviously the mapmakers are probably not going to charge you $30/h, but I would expect it still be a rate around minimum wage.