I am just looking at running some back tests on a selection method using betfair prices. What price should I use, is the WAP the one to target? Obviuosly its never going to be accurate (WAP price), but it should be a case of swings and roundabouts? Sometime it will be higher than the price you woudl have taken, and sometime lower? Would that be a fair assumption?

If not, what would you take?