Yes, but the US Navy builds them one at a time, every 5 years, so $2B/year and they are used to guarantee the cooperation of an important ally/partner with the USA instead of with Russia or China. There is a big cost, but there is a monetary benefit from the deals made by promising to always keep a carrier strike group near someone's shores, ready to defend from foreign aggression.
NASA's budget in 2015 was about $18 billion while the defense budget was just shy of $600 billion. If we lived in a fantasy world where NASA was considered as important as defense, we'd be an interplanetary species by now. Alas, we don't, and when we consider the feasibility of an idea we have to take that into account.