Consider only targeting Class 1,2 & 3 races only.
This will reduce the number of bets to a given day, this class seems to strike a number of $20+ mules along the way.

Maybe apply a filter such as , must have been $8.50 or less last start.
This tells us that it was not considered to be a totall dud in the market place last time out.

This rule does seem to work as well as anything else.

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Cheers.