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.

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