What is a fair hourly rate for an artist?
How long is a piece of string?
for some art - a great deal of skill and painstaking work goes in to its creation. There you will have a solid labor cost and that will largely be determined by the artist.
Above that, the value of the art is purely in the eye of the beholder. . . like any reasonable person, an artist will want as much money as they can possibly get for their work, but more often than not will have to settle for what people are prepared to pay them, which could be a huge amount if the artist has enough kudos.