Panna,
Could I suggest that your approach has an in-built flaw....?
By taking what the market offers you are always "betting to bad prices" and accordingly.....sorry mate but you will lose.....in the long term!
To alleviate this problem could I suggest that you avail yourself of your own ratings or follow those which are freely available on this website.
Thereby, you will not always be backing the general public "popular elect" which over the years has been found to be erroneous in up to 20-30% of all races.
That way you may regularly secure $6.0w or similar for some starters which are showing < $2.0win on the bookies' boards.
Hope that helps.

Havagoodun.