It's cheap because it's rapidly reusable. That depends on rapid reusability of the launch system, which still needs weeks of repairs between launches. And it depends on every component of the rocket working and not needing testing or significant refurb between reuse, which includes all 19,000 heat shield tiles. Rapid reusability is required to make it possible to do 15 refueling flights per starship mission beyond Earth orbit. Doing no more than 15 refueling flights depends on meeting payload specs, not yet proven. This is far from a complete list of even the known unknowns and their unknown solutions that each may need multiple iterations to establish reliability. Each iteration prior to safe return costs a booster and a Starship and 39 engines.

I encounter a lot of cases where people mistake Agile for "that's how people do project management now" and try iterative development when they shouldn't. Mostly in projects related to, but not directly involved in implementing software. This could be the most expensive case of "don't mimic how the software people do it."