All told, the cost of running the Death Star comes in at a total of about $7.7 quintillion per day. As Ovo points out, that’s 30 trillion times the amount of money across all of Earth. While that amount of money is hard to fathom in the real world, that’s the benefit of limitless imagination. Lucas didn’t need to consider the practicality of the Death Star in order to make it feasible, and certainly he probably never expected anyone to put forth the effort in doing the math.
That something else that science fiction inspires though. As a whole, the genre inspires fan engagement with the same limitless zeal with which it appeals to the imagination. Star Wars in particular is well known for inspiring conversation, discussion, and arguments among the fans. One could argue that this type of engagement is pointless, but it’s also an interesting way to put a realistic flourish on the fiction we love.
Just as the best sci-fi holds a mirror up for audiences to see themselves, so, too, can we hold up a mirror of our own. The perspective we gain on the realities of our favorite fictional worlds helps us better understand how those worlds operate. It adds a layer of perspective to the workings of those worlds, and in this case truly shows what the Rebel Alliance was up against. Anyone willing to spend $7.7 quintillion day on one station alone has resources far beyond the scope of a group of rag tag rebels. That makes their victory all the more stunning, and all the more heroic.