...While other manufacturers would demand both your kidneys for a 20-something inch monitor?
One argument I remember hearing is that the price would go up exponentially - up from 15 inches, but there are 80 inch 4k tv's coming out this year (for a pair of kidneys, true); On the other side of the pond, I hear people saying the opposite - that making small 4k monitors would be even more expensive, because it wouldn't be economically feasible to manufacture them at that high of a ppi.
So there are laptops with insanely high ppi, and 84 inch 4k tvs, which suggests that neither of the above is true.
Why, then, are there no reasonably-priced ** 4k PC monitors on the market yet?
** - Not to be confused with cheap; Just something that can be had for a few thousand dollars.
You're partially right, but also missing part of the picture. Monitor prices are not dictated by a single factor, so they aren't priced along a single continuum. As such there are two major factors driving monitor costs:
1) Monitor size. Larger monitors are of course composed of larger electronics layers, all of which need to be produced with few-to-no defects. Just as the odds of a defective GPU go up with die size, the odds of a defective layer go up with the layer size, particularly the thin film transistor (TFT) layer. Larger monitors also flat-out require more material to build, which also has an impact on cost.
2) Pixel density. Again with a focus on the TFT layer, the cost of manufacturing a display depends a great deal on pixel density. The greater your pixel density, the smaller the features of the monitor need to be, which is a challenge in and of itself. This impacts the TFT layer the most, as now you need more transistors, which means the odds of a defect just went up.
As a result, monitor prices are basically dictated by a pair of curves. One curve is the monitor size, which has a positive slope (rises). This mean that monitor costs increase with size of the monitor. Meanwhile the other curve is the pixel density, which when normalized for monitor size has a negative slope (decreases). For any fixed resolution, as you increase the size of a monitor the pixel density goes down and so does the cost of manufacturing. In other words, monitor costs decrease with the size of the monitor.
So to answer your question, the price of a monitor depends greatly on the nature of the price curves. If monitor production costs are more sensitive to overall size than density, then you'll be able to build a fairly small (but high density) monitor for less than you can build a large (but low density) monitor.
With that said, even with their larger sizes, laptop displays may just be at the edge of a production sweet spot. A desktop display wouldn't need to be quite as dense, but it's so much larger that the cost of producing a monitor that large starts to become an issue.
Finally, I would note that I haven't even gotten into economies of scale. 2600x1440 monitors aren't high volume items, so there's going to be some inefficiencies due to their low volume.