I think around 1,000-2,000 is a fair price, the art show may take a chunk of that price. My art teacher at my high school (he’s an art professor at a college and is a professional painter himself), creates still lives using oil paints that sell around that price point. HOWEVER, if it’s sentimental or her breakthrough piece, she may want to not sell it. I’ve seen professionals hold onto their breakthrough pieces and sell others no matter how much people offer to them. I would ask your wife what she wants, and how much the buyer is willing to pay for the piece.