It's every collector's dream: turning a modest outlay into a staggeringly high return on investment. ANTIQUES ROADSHOW "Jackpot" rolls the dice on those heady moments when luck smiles on flea market mavens and yard sale savants. This special recalls such spectacular windfalls as a landscape painting acquired for $1.50 at a Salvation Army "half price" sale and valued at $10,000 to $15,000; an art pottery vase, grabbed at a garage sale for less than five dollars and estimated to be worth $13,000 to $17,000; a 19th-century album of watercolor paintings, scooped up at a yard sale for a quarter, with an estimated value of $20,000 to $30,000; and a 1951 Minneapolis Millers baseball uniform worn by Willie Mays, scored at a sports collectibles show for $50 and valued at $60,000 to $80,000.