Give the artist $20 AND a free link/small banner on the site for a time period, say a year. Free advertising is worth its weight in gold, provided your site will have the traffic. But if that artist were to pay for the webspace to showcase a piece of their art, it would cost them more than $20, that's for sure.
People who want to ever make money have to see the BIG picture. Don't think small, and don't think immediate Forfeit instant moo-lah for name-building, branding, and recognition.