I estimated it would cost several orders of magnitude less, considering that the original estimate was based on Earth's primitive and costly methods of transporting material into orbit. Now some students at Lehigh have estimated the cost at about $850 quadrillion. Kevin Drum points out that this is far less intimidating than it might look. If we wait about 500 years to build it (at which point our technology might have advanced to Star Wars-like levels), this figure will only be about 65 times world GDP. Spread the project out over 20 years, and that's only 3 times world GDP. Moreover, spread it out over the wealth of tens of thousands of star systems in the Galactic Empire, and suddenly this is looking quite affordable.
Drum concludes from this that building the Death Star was "totally worth it." Well, that's not obvious. One must ask, compared to what? I mean, what was the point of the Death Star in the first place? To intimidate planets that might have considered undermining the Empire. It couldn't be everywhere at once, but the example of destroying Alderaan had to have served as a deterrent for other planets. Notably, the Rebellion had few options for bases after that. They stayed on an ice planet briefly, and after that had to wander space in a few random ships. But, of course, the deterrent value was limited, since the Death Star had been destroyed. Everyone knew the Empire could build another one, but until they did, the threat was going to wane. So they threw together another one, which again got destroyed.
Now, just as building a Death Star sent an important message to non-compliant planets, destroying a Death Star sent a powerful signal, as well. It was a huge public relations coup for the Rebellion. So the Death Star, while a devastating weapon, was also a tempting target. There were some basic conceptual design flaws (both versions were destroyed by a small smuggling ship and a handful of single-person fighters), but really, even if it had survived Yavin, it was doomed to spend much of its operational time fighting off attackers, simply by virtue of being such a big fat target.
Had the Empire instead used that money to build thousands of additional star destroyers, that likely would have been a much wiser investment. Then they really could be everywhere at once. Hell, park one in orbit around every system in the Empire. Then when Leia says, "They're on Dantooine," Tarkin can just call up the ship hovering over Dantooine and say, "Are there rebels there?", and the captain can say, "Uh, no. She's lying." Saves a lot of time. Plus, even if the rebels could down one or two star destroyers, that doesn't provide them with anywhere near the public relations value of taking down a Death Star.
So, on balance, I'd say it was a bad investment, even if an affordable one.
One other point: It was really stupid to put both the Emperor and his chief enforcer on an uncompleted Death Star right before an attack they knew was coming. Had Vader and Palpatine been back on Coruscant when that went down, the Rebels would have won an impressive victory, but the Empire wouldn't have fallen. It would have been like Al Qaeda sinking the USS Nimitz -- serious, to be sure, but hardly death to America.