This is taken from the DAS rules: (hope they don't mind)

"Favourite v The Field

Where you used to have to pick the winner to make your money, you now have the opportunity to LAY the favourite in a race by backing the field to beat it. To obtain a quote, simply request your operator for a market on the FIELD versus the FAVOURITE. Upon this demand, IASbet.com will quote you a price for the favourite and a price for the field against the favourite. This market will generally be set between 106 - 108%, while the total race market is more likely to be over 120%, so backing against the favourite puts you in a position of far greater value than backing every other runner at the best odds on offer. For Example "Favourite 6/4, Field 1/2" (or "Favourite $2.50, Field $1.50") means that you can either back the favourite at 6/4 ($2.50) or back any horse to beat the favourite at 1/2 ($1.50). In effect you are laying the favourite at 2/1 ($3). The only proviso is that if you call upon us to quote a market - you must have a bet, on or against the favourite. "

The 106-108% part seems pretty reasonble as you hardly ever see this sort of percentage bet over a full field or even in sports betting.

Just a thought, but if you're on the field @ 1.50 (66.7%) and the fav blows to more than 3.00 (33.3%) then you can wade in and back the fav and win no matter what.

I wonder if this ever happens?