You should look at the cost of ASICs (and where you can buy without getting ripped off, don't forget the import taxes), how to cool them (you said you're located in Australia), what is the cost of your kw/h and electricity contract etc. It's all a matter of scale, prices varies according to the situation.

Such a vast question can't be answered precisely. This forum is full of info, just have a look around and take the time to read.