They would measure it by using some sort of data reference to then determine if there was a significant statistical jump in, say, tourism dollars spent in Melbourne (this data is easily available to the department of treasury, i believe). Following that data, they would then see that the amount of dollars spent on tourism jumped by a significant amount above the normal inflation adjusted level for this kind of event. There are a few statistical tests to show when there has been a significant enhancement of growth above the normal average. Similar to how after the olympics, they calculate how much this contributed to a country's GDP, and its usually a positive investment (one would hope anyway).
I doubt his main audience were just locals tho. I would think that many people from interstate would have come specifically for Tiger (well at least a current affair found plenty of people stating their opinion for entry to the open was specifically for Tiger). So just interstate travel then you have these people spending money on local accomodation, local food, local restaurants, local public transport/cabs then of course the tickets to the open. So even that, its a significant number of dollars injected into the state, that otherwise would not have been there. I'm not sure if they include air travel in these estimates, but if they did then that would also add to the estimated 30 million injection.
So thats just the interstate input. the intra-state input may be similar as well (minus the travel) as there would be a significant number of people who would come to the golf that otherwise wouldn't. Then they spent money again on local food, local restuarants/nightclubs that then dissipates through the local economy.
maybe there is even an international factor as well, but again, i'm not absolutely sure what they include in their measurements. given Tiger's antics with his entourage staying at the most expensive hotel, he probably contributed half a million to the local economy too!!
originally they estimated Tiger's appearance to be 19million injected in the local economy, but then two months or so after the event when they got the data it was estimated to be i think 33mill, but not sure so i just said 30+ mill (it was definately 30 something)
so the jist of the very long post (sorry, a very very long post) is that paying tiger to come leads to more people visiting the state and spending money in the state, who otherwise wouldn't for this sporting event, and using that data they can then determine his impact on the local economy.