Tenacious,

First of all, if you can, you should try to make an assessment of how many lengths or kgs ahead of the field your top horse is. Take a look at the thread on this forum called 'Converting Ratings To Prices'. Might help you out. I'll show you an example from today using the pricing method I explain on that thread.

Here are the weight ratings I worked out for the Mornington Cup today:
8 Kounthekash 68
2 Charnwood Green 67.5
5 Fonome 64
3 Cool Trent 61
10 Highclere 61
1 Gunnamatta 60.5
9 Honest Politician 58
7 Garrard 57.5
13 Go Straight Home 57
11 Lord Of The Pines 55
12 Command 'N' Conquer 55
16 Bikkie Tin Blues 54.5
15 Bounty Beat 53.5
6 Big Pat 49
14 Babe Zaharias 48

Next I convert those ratings to points using a table I've created:
8 Kounthekash 100
2 Charnwood Green 90
5 Fonome 40
3 Cool Trent 18
10 Highclere 18
1 Gunnamatta 16
9 Honest Politician 8
7 Garrard 8
13 Go Straight Home 7
11 Lord Of The Pines 5
12 Command 'N' Conquer 5
16 Bikkie Tin Blues 5
15 Bounty Beat 4
6 Big Pat 2
14 Babe Zaharias 1

The total of all the points is 327. Now I divide 327 by each horse's score and round up or down to get a price:
8 Kounthekash $3.25
2 Charnwood Green $3.65
5 Fonome $8.20
3 Cool Trent $18.15
10 Highclere $18.15
1 Gunnamatta $20.45
9 Honest Politician $40.90
7 Garrard $40.90
13 Go Straight Home $46.70
11 Lord Of The Pines $65.40
12 Command 'N' Conquer $65.40
16 Bikkie Tin Blues $65.40
15 Bounty Beat $81.75
6 Big Pat $163
14 Babe Zaharias $327

One thing though. Unlike my example, you don't actually have to work out a rating or a price for every horse if you don't want to. If you are confident that you've sorted out the top 5 or 6 horses, you can just rate those.

Hope all that helps you out in some way. Any questions, just ask.