A BOUQUET OF IRISES sells for $20. A Van Gogh painting of the same flowers will run you about $50 million. Exorbitant, yes, but at least in commercial and artistic sequence. Of inverse order is "Three Decades of Ferrari," an art show hosted by Hollywood Sport Cars (through June 24). Prints of a Harold Cleworth acrylic of a Ferrari 250 GTO will be offered at a mere $300 apiece--while the real thing, a 1962 Ferrari 250 GTO, currently valued for between $10 million and $15 million. That any direct descendant from a used-car lot should be worth more than a half-dozen Faberge eggs is no surprise to Ferrari lovers or to Hollywood Sport Cars' Cris Vandagriff: "The mystique is in the lines, the curves, the sleekness."