okay I know this thread is super old, but whatever :D
I'm an illustrator just like -Ross and I can personally say that I'd usually charge around 1000 euros (around 1150 USD right now). This would take me 8-10 days, so I definitely need to charge that much to stay alive.
This is my portfolio for anyone who's interested:
https://www.annikamaar.de/For 300 USD you can hire a student or someone who just started out, but obviously then the risk of getting a piece that is maybe not up to the standards you envisioned is pretty big, just depends on how important this artwork is to you.
The "super pros" who work for AAA stuff like blizzard and riot get something around 5-15k USD for a splash art piece btw, at least from what i've heard