I like to take a double but the cost of 5 horse in each leg costs $12.50 for a 50 cent unit that returns 50 % of the div . does any one know of an online betting site that lets you bet on running doubles flexi betting for $6.25 for a 25 % return of the div ?

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.

how does this work?

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.

back to my first question today the running double on race 3to 4 in townsville paid just 10 $ and some cents on nsw tab but on supertab paid 31 $ if only there was a on line site that had best div on the super tab or nsw tote running doubles with flexi betting as well .

To moree--- I'm suggesting you not buy every Combination.
Only buy the Combinations that you consider are most likely of rolling up--- then I would not collect them if think there are 5 chance there are 5 chances in each leg