Thorns, I've looked at yesterday's results for Canterbury and Belmont. There were only 17 races.

The combined figures without Betfair's commission are:

Bet - a return of $130.65 on the Exchange.
Lay - a liability of $140.47 on the Exchange
S.P. - a return/liability of $141.27

From this, betting on S.P. had a 8% advantage over the Exchange. There was very little difference when laying. There were only 5 races where the Bet price was higher than the S.P. price, and 12 where S.P was higher.

Even though its only a sample of 17 races, and if I was betting not laying, I would seriously consider S.P.