I just did a random check of your theory. The first two races at Lismore today.
The order of betting in the first race was $2.80, $3.80, $4.60, $8 and $10. Third fav won at around the $4.60 mark. (I am using IAS prices around the second last fluc). Race 2, order of betting $3.60, $4, $5, $5.50 and $7.

The fav won at around the $3.60 mark. That would give you a return of $16.60. For interest sake the returns on the second leg would have been around $16.60, $18.40, $23, $25.30 and $32.20.

So in this case you wouldnt quite get your $20 back if the first couple won.

Interesting, hmmm, shall think on this more

Paul