RFG,
The comparisson isn't that black and white. Undergrad degrees (which include law and medicine) don't cost anywhere close to the figures you're talking about. International students might pay close to that, but the courses are still very heavily subsidised for Aussies irrespective of HECS. The most expensive degrees are postgrad ones such as an MBA, which cost about 40k from the better Unis.
Further to that, a medicine degree guarantees you a job. There are others that do too (teaching for example). First year doctors earn mid-40k depending which hospital you work at. Furthermore, if you're at a teaching hospital (all the big ones are), the extra training is provided as part of your contract. The alternative is to work at a private hospital with much better pay and conditions, but without the career progression. Incidently, 'apprentice' doctors generally constitute the bulk of MD staff in Accident-and-Emergency.
The only thing that balances private medicine or law practice against aviation is the rocketing insurance costs. But that doesn't apply to those professionals until later in their careers...