Glass deck above basement addition
Smells high, but hard to be sure. Older and presumably less expensive implementations seen under sidewalks use more of a concrete grid with small pieces of frosted glass - better relative strength and modesty preserved, but no stargazing from the basement. And, if the glass was broken (it's usually quite robust) there's no chance of people falling through it, given the much tighter support grid.

I'd imagine the sort of thing pictured is done on a one-off basis, so you have to pay for the engineering to your specific application, and the materials. And probably someone's liability insurance on the thing.