I think what Mark may have been getting at is that the priceline can easily add up to more than a 100% market.

For example if you have 3 horses in a race. Each with a strike rate of 50%, therefore by your method the fair price of each is $2 or even money. So then if you could get odds of $2.5 for each runner (120% market) you would then bet each as by your method they are value. Yet clearly they are not.

Therefore you need some way of making the percentages add up to 100%.

Cheers,
Blocka.