Um, it's cause you live in Australia

Dunno if it is due to your import restrictions or transport costs or hardness to breed or all of that

They're not cheap here either, depending on the species, but nothing compared to your prices. Some are widely bred here, but many still have to be imported from South America or maybe from Asian breeders which raises the price.
Some of the most wanted here I think are zebra plec*s (only few breeders here so far) and Panaque suttonorum which are quite rare and therefore expensive.