The amount that you can bet depends on the track category and on the bet type you choose. For example, a Straight bet on a Category A Track lets you bet a minimum of $1.00 up to $1,000.00; an Exacta on a track of the same category will let risk from $0.50 up to $500.00.
These details are visible when you are in the Racing section at the bottom of the screen when you are placing a bet.