The Labor government allowed the stratospheric AUKUS cost figure to enter the public discourse because it is setting the stage for political fights to come with the Liberal party. In crude terms, Labor is going to argue for increased taxation to fund AUKUS, in particular scrapping the recent "Stage 3" tax cuts that are estimated to reduce government revenue by AUD $120bn over the next decade. Labor is going to use the cost of AUKUS as a talisman of its commitment to Australia's national security (neutralising a typical line of attack from right-wing parties) and as a club to advance its basic ideology. Labor will probably put reversing the Stage 3 tax cuts on the table at the next election and dare the Liberals to publicly argue for (a) less national security, (b) more "irresponsible" deficit spending or (c) cuts to popular categories of spending such as health, education, etc. instead.
More broadly, folks consistently underestimate the role of domestic politics in shaping statements and policies on foreign matters. Most relevantly, American attitudes, statements and policies towards China have very little to do with China and everything to do with domestic politics in America. There is some relationship between the outside world and the domestic discourse, but it is greatly attenuated if not outright distorted by the "Rube Goldberg" black box that is the domestic political apparatus.
The more powerful a country is, the more the domestic discourse predominates over the external reality. For a small, weak nation, the costs of a large divergence between internal discourse and external reality can be immediate and severe. For America, decades of policy failure towards Cuba, Iran, North Korea etc. has no discernible effect on the discourse because America's power shields it from the consequences of failure. Political discourse in America regarding Russia, China, Cuba, Iran, Ukraine, Israel, etc. cannot be understood by reference to those countries, rather they are simply symbols tossed around by players in what is overwhelmingly a circular-internal debate about what it means to be America.
This is one of the reasons why I disagree with the optimists who argue that catastrophic conflicts are unlikely because nobody wants such a conflict and will therefore not act in such a way as to precipitate it. This ignores the aforementioned tendency for the statements, policies, actions of powerful nations to be only distantly related to the external reality as experienced by the other.