Has anyone got any figures on how this figure holds up compared to taking odds on the market close to the close of betting for a particular race?

Is the SP generally higher or lower, or does it even out over time?

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.