Yes, Septillion, that is a 1, followed by 24 zeros.
A very, VERY large number, especially when it concerns money. And why is it a deal? Because that figure ignores a lot of very important costs. Costs for what? Why to build a Death Star of course!
Via my good friend Hoade, “One Death Star for $15 Septillion? What a deal! [1]”
Not quite. Ryszard Gold there seriously overcalculated the price of the Death Star, by several orders of magnitude.
First off, his figure of 17,157,284,678,805,056 cubic meters is too high, and I think I know his mistake. The volume of a sphere is (roughly) the radius cubed. Ryszard used a figure of 160km (or 160,000m) for the diameter, and cubed that (160,000m) instead of the actual radius (80,000m), so his Death Star is 8 times too large. It's actually 2,144,660,584,850,632 cubic meters.
So his cost of the raw steel, $1.3×10^19, is too high—it's closer to $1.6×10^18—an order of magnitude cheaper. But that's the cost of steel made on earth (excluding shipping, which is $1.2×10^25, or $12 septillion dollars). But if you're shipping that much steel up to Earth orbit, you're doing it wrong. It'd be way cheaper to ship steel in from the Asteroid Belt than to lift it to Earth orbit. Or heck, cheaper still to find some asteroid and build the Death Star right there. It'll only take a medium sized asteroid.
Taking Ryszard's figure of 1/10 the mass as structural, and assuming that asteroids are 80% pure ore (which isn't all that unreasonable) then all you need is an asteroid 80km in diameter. Plenty of candidates to choose from [2]. And it wouldn't surprise me if there aren't a few, or a score, of asteroids made out of CHON (Carbon, Hydrogen, Oxygen, Nitrogen) [3], so no need ship air or water up from Earth either.
So yes, the price tag of the Death Star is up there, but nowhere near the $1.5×10^25.
[1] http://rickgold.info/ds/Site/Welcome.html
[2] http://en.wikipedia.org/wiki/Asteroid_belt#Characteristics