If all 20 had started, then you would need $2 * 20P4 = $2 * 20 * 19 * 18 * 17 = $232,560. For 18, it's just $2 * 18 * 17 * 16* 15 = $146,880. Keep in mind that while you would win this time had you bought all the tickets, most of the time you would come out way behind due to the nature of parimutuel betting. You get a huge chunk of change for this superfecta because the favorite finished last.