Shags, there are much higher costs in GA. For a GA flying school to operate they need an AOC, which could cost $10,000 or more. They need an Ops manual which must be approved by CASA, and CASA charges an hourly fee. They need CASA to approve a Drug and Alcohol Management Plan. To operate as a flying school they need a CFI, and the qualifications required for a GA CFI are very high. CASA must approve a particular person to be a CFI. If the CFI leaves, the school cannot fly, they must shut down until CASA approves the new one. Because GA CFIs are highly qualified, they are usually quite well paid. The amount the school can do depends on these qualifications, so a CFI might only be approved for the school to teach to PPL for example.
The aircraft are not in private category, they must be in the minimum of airwork category and may be in charter. They are maintained by qualified LAMEs, who charge a high hourly rate for their expertise. All the aircraft are on a particular maintenance schedule and it isn't cheap. Add required instrument checks and ADs, routine maintenance for aircraft flying at night or in IMC. The instruments in that case are required to be certified to a particular standard, and they are expensive. IFR GPS units have databases which need to be updated monthly and the subscriptions are expensive.
The purchase price is higher. If you are talking about new, a 172 would be about $300,000. Most schools these days have a mix of old and new, some all new, but those aircraft really were built to last.
The fuel costs are higher, bigger, heavier aircraft with bigger engines use more fuel, and they use aviation fuel.
Even the most basic of the GA instructors does 50 hours of flying for the instructor rating (more than double the RA requirement), and they do this on top of a CPL and NVFR or CIR. So each instructor has outlaid many tens of thousands of dollars to train you. And all of them pay for Class 1 medicals every year.
It is because of all this money spent on training, aircraft certification and maintenance, that you are able to do night training, instrument ratings, aerobatics, multi-engine and so on. Also remember the instructors need additional approvals to be able to teach some of these, so that again means more money.
Operators flying from the bigger controlled airports have higher costs for hangarage, parking and landing fees, or if flying IFR, for enroute fees. Landing fees are based on weight, so even if RA operators are at the same field, the landing fees are lower. It even comes down to hangarage. Simply due to size, it may be possible to fit more RA-Aus aircraft in the same hangar space.
Then in the training, due to the syllabus, you need to fly more hours for your training, again adding to the cost.
In a nutshell, it is not a level playing field.