Something for everyone by John P. Sparkman
"Table 1 summarizes data from the ten most important sales of two-year-olds in training - alphabetically... for horses sold at those ten sales from 1997 through 2006. Remarkably enough, the 23,333 horses sold at those ten sales over that ten-year window have earned back 95.2% of their $1.45-billion total purchase price. That is a far higher percentage than graduates of yearling sales manage."
This does not take into account the horses that RNA or did not make it to the sale, or the residual values of the horses after racing.