Am I missing something because $50 Aus Billion is about $40 B US or $3.33 US billion per sub. That is awfully expensive for conventional subs.
It is, but there are a few reasons for the costs that go beyond just building the boats. That money includes upgrading port infrastructure to handle them properly, upgrading facilities for building them (which will also then be available for other projects) and increased costs associated with building the submarines in Australia - it would be cheaper to have them built elsewhere but this ensures that warship building skills within Australia will not be lost - once you lose those skills and industries, it becomes much harder to get them back and that's worth a certain premium.
ASC (who will be building the submarines) employs two and a half thousand workers - they are currently building our Hobart class air warfare destroyers, but that work will start scaling down next year and be complete by 2020 - this deal means that industry and its skill base will survive.