Nothing they maufacture all parts/materials themselves using replicators
Also since American currency doesn't exist in the 24th century it's almost impossible to come up with a number... One has to consider the technology used to create many parts and materials needed don't even exist yet so it's anyone's guess
Since energy reserves are theoretically infinite via fusion or the like, and that energy powers replicators which can manufacture anything they need, I would say that it costs nothing.
In the Star Trek: The Original Series episode "Mirror, Mirror", the Enterprise is referred to as a "fifty-million-credit starship", suggesting that was the construction cost of the Enterprise. The subsequent Enterprises would have cost more, adjusted for inflation ProfessorTrek 03:50, August 4, 2011 (UTC)