I bought a Bond 'kit' type cage with all the bars bent ready to cut and weld in and got a mate that builds cages down here to weld it in for me on the cheap. It only has the basic bars required by CAMS but it did make a big difference to the handling. I pulled about 1.5 seconds off my lap times straight away. We're now going through the process of adding a cross bar behind the dash and linking the cage into the front strut towers. The kit from Bond was about $1,000 but that's going back a few years now.
To get a workshop to do it, $2500 will be on the cheaper side but you might be able to find somebody who will build a fairly basic cage like mine for around that. Expect to pay anywhere from $4-8K+ for a fairly serious cage built from CDS, more again for chrome moly.
Depending on the workshop, you will probably be able to save yourself a bit of cash by having the car ready to go so that they don't have to remove and re-install anything. Interior, dash & windscreen out, etc, etc.
That's all i can think of for now, hope that helps.
I'm sure dat2kman & PZG302 will be able to point you in the right direction for some cage builders in SE QLD.