Ok, I haven't cross checked the source of your figures but are you comparing the through life costs (say AUS$120B) of 12 Attack class submarines against only 3 Virginia class (AUS$360B)? If so, you think 3x SSN are 3x better than 12x SSK??
I believe that life cost estimate isn’t for just the initial 3 second hand Virginia boats but includes the cost of 8 AUKUS boats* - 5 to supplement the Virginias and the final 3 to replace them. (And the total also includes a hefty $130B in contingency funds)
Though, by that stage (around 2060), with Australian production in full flow, it may switch to whatever is the then planned AUKUS successor.
*
https://www.abc.net.au/news/2023-03-...nced/102091510
https://www.nti.org/analysis/article...-capabilities/