The primary cost in replacing the undersea nuclear deterrent is the replacement of the actual submarines themselves. Submarines, like aircraft, can only be pressurised and depressurised so many times before thier hulls become too weak due to metal fatigue. Also the equipment inside gets old from use / obsolescence / wear and tear. Also a submarine's greatest weapon is its stealth, not being detectable while submerged. Improvements in sonar technology mean that older submarines are losing that edge. There's not much point in having them out there at all if your enemy can pinpoint them and destroy them before they can launch. That's a Boomer's whole point is the retaliatory strike capability. Kinda hard to retaliate if you're dead. Also something to consider. The reactors that they run on are only good for so long before the metal in the hull surrounding them becomes brittle. And believe it or not, it's MUCH cheaper to build a whole new submarine than it is to try and swap a new section of hull with a new reactor into an existing sub. Recycling of nuclear submarines is a VERY expensive hobby!! It is also the fact that not only the US has these scary behemoths out there that keeps countries like North Korea from completely flexing thier muscles and building stuff to lob at Taiwan and Tokyo. These governments realise that even if they completely decimate the entire United States that the UK, Australia, Canada, etc have the capabiity to turn them into a glowing glass parking lot. Plus you get countries like Japan, India, Norway, Mexico (to name a few) that have non-nuclear ICBM's laying around in the oceans that will (under certain circumstances) back up any of the other above mentioned nations, and you begin to see that it's a great big brotherhood of "us" vs "them" that keeps everyone in check. A nuclear submarine and its armament loadout can be desiged, assembled, tested, and loaded completely in secrecy. Yes, even in this world of google earth and such. Just ask anyone about the Seawolf program. There's STILL no info available about it, even though it's already sailed (I lived in Bremerton, Washington and drove taxi to Bangor Submarine Base in Bangor, Washington many times) since BEFORE 1998. So if the Yanks can do it in a fairly public area of the world, I think the North Koreans could do it relatively easy considering how much they supress thier people. And the only weapon you can use against an SSBN carrying nuke ICBM's? That's right - the threat of retaliation from an opposing SSBN carrying nuke ICBM's....... So long as there are countries out there like North Korea that are willing to thumb thier noses at world organisations like NATO and the UN, worldwide useage of SSBN's and thier deadly cargo is a necessity. Whatever the cost to the economy. It's a LOT more expensive to try and restore London to a habitable state after a nuclear strike against it.