[H]ow big is the Republic/Empire? There’s probably a canonical figure somewhere, but I don’t know where. So I’ll just pull a number out of my ass based on the apparent size of the Old Senate, and figure a bare minimum of 10,000 planets. That means the Death Star requires .03 percent of the GDP of each planet in the Republic/Empire annually. By comparison, this is the equivalent of about $5 billion per year in the current-day United States.
Went there first, I did, but not in as much detail.