At the moment you are purchasing 5 horses in each Leg , therefore there are 25 different Combinations in your Doubles.
It seems your dilemna is that you can't afford the $12.50 to buy every combination.
I'm suggesting you not buy every Combination.
Only buy the Combinations that you consider are most likely of rolling up.

Say you only had 3 animals in each Leg.
There are then 9 permutations.

Top Pick into Top Pick
Top Pick into 2nd Pick
Top Pick into 3rd Pick

2nd Pick into Top Pick
2nd Pick into 2nd Pick
2nd Pick into 3rd Pick

3rd Pick into Top Pick
3rd Pick into 2nd Pick
3rd Pick into 3rd Pick

The Combination Most Likely to win is Top Pick into Top Pick
The Combination Least Likely to win is 3rd Pick into 3rd Pick

In this Example I would suggest eliminate ALL Combinations that do not contain a Top Pick , which would narrow the Combinations from 9 , down to 5.