I can understand what you are doing is playing the percentages but as you have said this will only work up to a particular price.

The 30% is based on every fav that wins including those at $1.40 to as high aswell $6+ what you need to do is look at the price range that you can actualy make a profit at and work out the percentage from there.

using your example if you lost 65% of races your profit is $2,372.50
and winning 35% of races gives you a loss of $2,222.50
$2,372.50 - $2,222.50 = $145.50 - 5% commission = $138.22

Now this is not all that bad if you multiply the amounts by 10 but you can see where i am going with the percentages depending on the prices..