Oh, my my God, 500 km diameter circle can cover many cities inside! My Chinese client only tried me to consult on a 100 km^2 or so project, I not yet refuse, but probably will refuse soon, because of too low power density.I first heard of the solar updraft tower around 1995, when a developer in India asked my firm to do an engineering evaluation of his design, We declined because the proposed scale (500 km diameter at the base) was far beyond engineering experience. But I recall the principle that efficiency of an updraft tower increased with physical scale.
Do you know Moore's law in IC industry? I don't believe the transistors density increase endlessly, and probably current CPUs already touch the limit.
Same thing with PV solar panels, I don't think the price can drop down in large scale, but may subject to minor decrease.
I know how the raw silicon materials are processed into usable PV cells, the processes consume phenomenal energy in crucible pots, therefore its price is hardly to further drop down, as the processes may be already tuned for max optimization.
Above just my personal opinions.