You would count all moneys bet on each race.
Ie $10 on first race - wins paying $5.
$50 on 2nd race - wins paying $3
$150 on 3rd race - hopefully wins.

Turnover is $210. Its the same as if you placed individual bets of $10, $50 and $150 on those 3 races. The only difference is with an Allup you are betting with winnings, which may or may not be the case with the other bets.

While you only risked $10 of your own to begin with, your actually turned over $210 in bets, by risking your winnings plus original $10 each race.