Welcome to the forums!
It's a bit tricky to give you a good answer to this question because it varies so so much as to where you live, who you see and what they specialise in or have a niche in.
For me personally, the most expensive session I have seen is over 300$. The APS (Australian Psychological Society) sets a recommended consultation fee at 251$ From what I understand, the psychologist has the freedom to set their own fees (although I could be wrong).
The cheapest session I have seen is, well, free. There are psychologists who bulk bill or work for community centres and choose not to charge, even with mental health care plans.
It's important to know that mental health care plans don't always give you the sessions for free. The rebate depends on the individual psychologist and their own fee policies.
The difficulty is that finding out what psychologists charge and their medicare gap is not easy. This website may list some bulk billing psychologists if you enter your location and filter by 'bulk billing' on the left hand side. - https://www.psychology.org.au/Find-a-Psychologist
Hope this helps,