Ridiculous estimates like those only get thrown out when the people doing the estimating don't actually want to build the thing.
A new stadium in Canberra anytime in the next couple decades is dead, and has been for a few years now.
The only way the Raiders and NRL could change that is if; A. they convince the Federal government to step in (fat chance), or B. they play hardball and do something radical, i.e. threaten to relocate the team as that's really the only card they have up their sleeve that could give them any leverage over the government at all.
Aside from the two options above they're basically left with finding a way to build their own stadium, which is a pipedream at this point, or hoping and praying that a Liberal government gets voted in sometime soon, which is significantly less likely than the Feds stepping in.
BTW, isn't it curious how inflation hasn't significantly effected the costs to upgrade Manuka LOL.