Non-finishers

Anyone able to point me to some statistics on how often horses start, but fail to finish races? Both flat and National Hunt.

I understand that bets on such horses stand (and naturally settle as losers). This means that, in spreadbetting, on say a three horse race where the index is 50-25-10, it's possible for a horse to settle as zero if it fails to finish. So the theoretical prices for the three horses should add up to less than 85 because of this risk. But I don't have any handle on how much less.

Thanks.