I might be wrong but I've noticed over the past few days the S.P. price has been identical to the last price matched on Betfair. However, I've only checked a few random horses when their S.P. price flashes up on the screen during the running of the race.

If this is really the case where the S.P. matches the last price traded, then that could be somewhat risky for Betfair especially if/when there are distorted amounts in either the Buy or Sell sections of the S.P.