Betfair prices need to be taken with a grain of salt as the price I get is probably going to be differnt to the price you get. I might get $2.00 and you might get $2.20 all within 30 secs of each other.

As far as people using tab prices and assuming 20% or whatever that is the biggest joke ever. I have seen horses start at under $3.00 on the tab and over $5 on betfair, and I have also seen it the other way round. If your going to bet on betfair use their prices for testing. And don't use the averages as you will rarely get the weighted average anyway. If you don't collect your own prices then you are walking around in the dark testing against any average prices.