That is true. Money gets weird at huge scales. For something like an interstellar mission a more significant limiting factor may be the number of engineers and skilled workers with the right skills to work on such a thing and the rate at which they can be mobilized and managed. Most such people are currently employed, so they would basically have to be poached at huge scale. It takes a long time to train people like that, so the supply is inelastic on short time scales.