A freelance artist ususally starts at $25 and up, anything below that number and you are probably with a newbie artist or someone who really wants to compete with the old guys, in short never a good idea to devalue your work...
You should look into royalties, advances + splitting works(%), if you gave her an advance, then she will probably not get any money from the beginning - when the game is published, until you earn at least that amount back...
It's basic negotiating...