Not so much low lost as one-trick-pony.
Bombers can carry a large variety of munitions. The 747 concept was limited to ALCMs. Where each rotary launcher was chucked out the back once it’s missiles were expended.
In the nuclear role it was a one-shot doomsday platform, a role that was already being adequately filled by SSBNs and ICBMs. In the conventional role, with the then-secrer CALCMs,it was a single use, one shot first-night-of-war platform, lacking the “day two” flexibilty of the other strategic bombers.
Low cost, that is.