CAREERS in sports tend to be short, and most professional athletes hope to earn enough during their playing days to enjoy a long and comfortable retirement. Human ones, that is. For the equine athletes, the big bucks start when the racing stops. On October 20th Frankel, a four-year-old bay thoroughbred, won his 14th and final race. Timeform, a handicapper, has given him its highest rating ever. During his undefeated career, Frankel earned £3m ($4.8m) for his owner, Khalid Abdullah, a Saudi prince. He could make the same amount in just a few months once he is sent to stud. A typical stallion can mate with 100-150 mares a year for a decade, and Mr Abdullah is expected to charge at least £100,000 for Frankel's services. If his foals perform well once they start racing in 2016, that amount could triple. On the surface, £100,000 might sound like a bargain. Promising yearlings (horses aged one to two) sell for some £500,000, and an owner could earn back that sum by winning two of the high-stakes races at Royal Ascot. However, paying for upkeep and jockeys does not come cheap. And good genes are no guarantee of success. Just 59 of Frankel's 218 half-siblings who have raced have earned back the £120,000 stud fee charged by their sire, Galileo; 20 failed to win anything. High Chaparral, a stallion who shares an owner with Galileo, proved better value. His foals have earned an average of £167,000 - only slightly less than the £179,000 won by an average offspring of Galileo - and his services can be had for a mere £20,000. Nonetheless, few owners of top racehorses are in it for the money. Many will pay up just to boast of their animals" bloodlines. Frankel "is a collector's item, like a rare work of art," says Johnny McKeever, the owner of a breeding agency. And even if his offspring fail to replicate his success, they can always be sent to stud themselves to help earn back their cost.