mate,
the first thing i reckon to do, is order the horses from first to last in the order you think they will finish

if you can pick out say half the field you think "can't" win price them anywhere from $101 to $9999 and it will cost you vitually nothing, then start thinking how strong is your winning chance, put a price to it, then just go down, as stated earlier in the thread (i use google) just do 100/(the price) and add them all together,

ie
5 horse field
Defier $2.30
Lohnro $2.80
Fields Of Omagh $7
Makybe Diva $21
Dandy Kid $101

100/2.3+100/2.8+100/7+100/21+100/101
= 99.2
so your market is framed to 99.2%