Daily Profit Objective Per Amount of Bankroll

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Like what Gandalf said above it depends.
The contributions to this question would be:
Hit rate (How many times your bets hit at)
Wager amounts (How much on a Win bet, Exacta, Pick 3 $1, $2 $50 etc)
Odds (How big of a price is he/she? 3/2,1/2,1/1,5-1 etc)
Bets per race (Bet a lot per 1 race or bet every few etc)
Amount Won Back From Winning Bets (Kind of like #2)

Those factors above play a big roll in how much profit you have with $500 and how far you can stretch it over races over time.

Hopefully this helps. Good luck ScottB!