You are purchasing each Combination for equal amounts when you do this.

What you could consider is placing the animals in Order of Best Chance of winning each race.

Say you fancy in Order For 1st Leg 2,1,5,7,9
And for 2nd Leg 3,1,5,2,8

Try eliminating some Combinations to reduce your Outlay.
Take these Doubles.
2 - 3,1,5,2,8
1 - 3,1,5,2
5 - 3,1,5
7 - 3,1
9 - 3

That'll cost you $7.50 for Half a Unit.
It includes the Combinations you figure as having the Best Chance of rocking up.