Estimate how many people you would have needed to employ and what their salaries might be, estimate the cost of materials...
But those aren't the main reasons for the costs. The main reasons are that rockets have not benefited from reduced costs through mass production - something that would be very hard to estimate if you didn't know or guess that.
There's a lot of background mess in our mental pictures of the world. We try and be accurate on important issues, but a whole lot of the less important stuff we pick up from the media, the movies, and random impressions. And once these impressions are in our mental pictures, they just don't go away - until we find a fact that causes us to say "huh", and reassess.
Here are three facts that have caused that "huh" in me, recently, and completely rearranged minor parts of my mental map. I'm sharing them here, because that experience is a valuable one.