This is how my system is developing.

Starting from the bottom weight,put the runners in quickly considered finishing order,from first to last.
The last horse gets a rating of 1.
Then after looking closer at the form details,give each runner a rating compared to the runner below it,working your way up from the bottom of the list towards the top.Try to give ratings that convert to prices that look about right.
Once it looks like it can prove worthwhile ,I'll chuck it in a Excel attachment.