Daily Racing Form had a story Pre-Pandemic after the California tracks went thru some negotiating with Internet Wagering companies.
Pretty much said that tracks get between 3-5% from off track. More or less depending on the need for your signal.
On track is setup by state with different takeout percentages for different types of bets. You will see some advertising that a pick 4 or other exotic wager is boasting a smaller 15% takeout.