Have you ever played piano before? If not you can look for a beginner teacher (maybe an AMEB examiner). They could charge anywhere between 30-60$ for 45 min lessons.
You don't have to do every single grade, ofcourse you can if you want to but if you want to save a bit of $$$ and instead of doing the same thing over and over again for each grade, you can skip some grades. However you can try grade 1 first maybe just to see what a piano exam is like. Or you can just play pieces from grade 1, 2, 3... etc and then do a higher grade. When I began piano, I played grade 5-6 pieces but never did the exam, the only grade I did was 8.
Moving up the grades depends on how fast your skills develop etc, so really it varies from person to person, if you are thinking about taking piano for fun you can do the leisure exams.