Here is a pricing range based on Ranking order that seems to be as accurate as anything else. Rightly or wrongly.
Based on a market of 118% as per TAB market.

It is also based on the loose assumption that the majority of winners will be selected within the Top 7 selections.

Top 7 only.
Ranking
1---------$3.80
2----------4.00
3----------5.00
4----------7.00
5----------9.00
6----------12.00
7----------16.00

Please keep in mind with this type of approach, is that the short priced Fav is your biggest enemy , one will hit some great prices so as long the Fav does not decide to get up all day, one should do well.

One approach is to target races where the Fav is starting from Barrier 9 outwards , they often fall over when in this position.
It also narrows the number of races down for you aswell.

Cheers.

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Cheers.