Did you know that Australia has a few private use light aircraft manufacturers (suitable for VH registration and PPL) and that there are many around the world that either modify or make PPL required and "VH" register-able "KITS" specifically for amphibian landing/takeoff private aircraft.
More to the point, if you look at the price of Cessna and Piper e.t.c. they are generally around 300K USD to 500K USD but you can get pretty good kits in Australia and many other countries that do much the same at between 100K and 200K including some amphibian !

Usual price is "around" give or take 100K AUD (80k - 130k AUD) for a kit or factory built light aircraft (land rolling).

One of the greatest recent features since 2013 is manufacture of cheap kits for short take-off landing (STOL) light aircraft that can land and take-off over "no more than 60 meters", a feature worthy of building into amphibians, but with PPL and VH registration you can keep it in a hangar on a small block out of town (obstacles permitting).

Did you know?! You do not need a PPL or RPL(recreational - one of the most useless levels of license possible) license to fly any light aircraft outside controlled air space up to 8000 feet altitude (e.g. official Aerodromes), its draw-back is major airports is 50Km radius and regional are around 25Km radius and there are military bases to not be within particular distance of!
note: That is why in the country areas you sometimes see low flying "full size" light aircraft with no registration markers !

About Australian light aircraft use (PDF document).
windsolarhybridaustralia.x10.mx/PPL-light-aircraft.pdf

Highlighted STOL kit aircraft


Professional manufactured STOL example


Kit amphibian aircraft