betfair SP
 

Does anyone have a file of all BF SP for the last year or so on aus racing

Its available at betfair.

http://www.betfairpromo.com/betfairsp/prices/

Its interesting to note that SP prices will change as you add money to one side of the equation, similar to the totes. The more money you put on a selection to back it the lower the SP price will be when you start betting. The more you put on to lay the selection then the higher the final SP price will be.

Does anyone have anyway to work out the affect of a bet on SP before placing it ?

I am obviously not talking about $5 bets here but $1000+ bets/lays. I would be interested to see how others are handling it.

It's slightly more complicated than that. There is an algorithm that is based on the weight of money at certain prices close to the last matched bet, but not the last matched bet I believe, and money waiting to back and lay at SP.
There's a bloke on another forum, who worked there at the time when they were working on the algorithm, and even he had trouble deciphering exactly how it's done. He did explain there's a complete set of number crunching calculations going on at the same time to arrive at the final SP price.

I'd suggest that $1,000 wagers on outsiders would effect the SP.
I'd also suggest that $10,000 wagers on anything in the market would not, for the reasons above, otherwise it would open massive arbitrage opportunities for those with massive resources, for the reasons outlined above. That's why the calculations are so complicated.