I suspect that the K bikes are at that awkward stage where they are just old enough to be "old" but not old enough to be "vintage".
Yes, they are cheap because the brick engine is so strange. The resultant low price due to the ugly duckling nature of the brick is what makes them so attractive to those looking for a cheap machine to modify, but in the end, that is what dooms the value of what they build. Cafe bikes by definition are carbureted singles or twins, with spoke wheels, a five wire harness and a British nameplate. The K bikes have none of that, and as a result won;t survive in the cafe bike marketplace.
As much as I decry the cafe craze, I must in the end be grateful for the "builders" who are turning these fine machines into future derelicts. By making them uncomfortable to ride and overly hip stylingwise they virtually guarantee that these bikes will be left for dead in about three years. If we are lucky, they will be abandoned to an indoor space away from the sun and rain to collect a lot of dirt and rat shit providing a future source of somewhat useable driveline spares. In addition, the agressive thinning of the herd will make the value of well maintained bricks that much greater when they finally achieve vintage status.