The Death Star is a surprisingly cost-effective weapons system

First off, the technology of the Star Wars universe is well in our future. How far into our future? Well, Star Trek is about 300 years in our future, and the technology of Star Wars is obviously well beyond that. Let’s call it 500 years. What will the world’s GDP be in the year 2500? Answer: assuming a modest 2% real growth rate, it will be about 20,000 times higher than today. So we can figure that the average world in the Star Wars universe is about 20,000x richer than present-day Earth, which means the Death Star would cost about 65x the average world’s GDP.

However, the original Death Star took a couple of decades to build. So its annual budget is something on the order of 3x the average world’s GDP.

But how 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% 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.