This Page
https://www.sara-tx.org/public-infor...past-projects/ has a lot of information on The San Antonio River flood diversion tunnel. At 3 miles long and over 24 feet in diameter this is a really good model for the manned bases with a frozen team I've been working on. This project cost over 110 million dollars. This is also about the smallest size for Prime Base, if the test campaign with three levels is used as a benchmark. Camp Century had around 2 miles of tunnels and 200 persons on staff, for another benchmark.
From this we can see that a Project with a dozen bases, perhaps including Prime and Alternative Prime would cost in the low billions of dollars, maybe 10 billion as a cap.
Project Azorian, which was the attempt to raise Soviet submarine K-129 cost about 800 million dollars (in 1970s dollars not 1990s dollars). It is around an order of magnitude less. From the sinking of K-129 in 1968 to the revealing of the story by Anderson in 1975 was around 6 years. Not only did the US media have information but the Soviets did as well, although their experts believed the salvage was either impossible or highly unlikely and they didn't act aggressively on that prospect.
I'm not sure what this says about budgets and costs and operational security but does at least give some figures for period projects.