Cost effective is a relative term when it comes to military hardware, especially if you are talking about cutting edge technology equipment. The B-70 you mentioned was an incredible aircraft for its time, but was bombastically expensive and never saw production because precision guided missiles rendered it obsolete. Manned nuclear bombers are of questionable value in this day when drones and cruise missiles can perform most tasks without endangering any crew members. When you have the largest and most sophisticate military in the world, you are going to have lots of things that cost an astronomical amount of money. The real question is whether it fulfills a critical need that justifies its cost, both in terms of acquisition and ongoing maintenance and support. I agree that the B-2 is certainly on the fence in that regard.