Hi,
Here is what Betfair say about how their SP is calculated:-
Unlike other bookmakers’ Starting Price, the Betfair Starting Price will be determined by balancing bets from customers who want to back and lay, so because it’s other customers placing the bets and because there is no margin for profit built in, you are far more likely to have better odds than the bookies’ SP.
The Betfair Starting Price will be calculated exactly to ensure the fairest and most transparent odds possible for both backers and layers. The Betfair Starting Price does not need to account for a profit margin but instead is calculated at the start of an event by looking at the relationship between the amounts of money requested at SP by opposing customers and by looking at unmatched Exchange bets.
Regards,
Mark