How expensive has defence procurement become in the US?

'A lot' appears to be the answer:

According to the Government Accountability Office, cost overruns have ballooned to more than $450 billion over the past two decades. The Navy needs to take authority back from the bureaucracy, end the culture of constant design changes and gold-plating, and bring back fixed-price competition.

Recall the development of the Polaris nuclear-missile system in the late 1950s. The whole package—a nuclear submarine, a solid-fuel missile, an underwater launch system, a nuclear warhead and a guidance system—went from the drawing board to deployment in four years (and using slide rules).

Today, according to the Defense Business Board, the average development timeline for much less complex weapons is 22.5 years. A case in point is the Ford-class aircraft carrier. The program is two years delayed and $2.4 billion over budget.

and more:

Yet the defense firms involved still profit under cost-plus contracts. The three stealthy Zumwalt-class destroyers—they are really heavy cruisers—are another example.

The defense bureaucracy produced a seagoing camel costing three times its original estimate and delivered with questionable seaworthiness and without functional radar or a reliable propulsion system.

Both quotes come from John Lehman, US secretary of the Navy under President Reagan.

Note the push-back against cost-plus contracts and how they provide economic operators with the incentive to keep the costs ticking even if they are not necessarily making a profit (helps soak up capacity, cashflow management etc).