Well, yes, NASA does rely a lot on external contractors due to the budgetary flexibility it entails. While there is some cross-communication between teams, it's generally limited by the extreme degree of specialisation needed. Also, as a general rule almost all of the hardware for any given mission is custom built for that mission, so the necessary knowhow to operate it is both unique and irreplaceable.
That said, some of the extra cost is due to scope creep, or due to changes in launch vehicle availability requiring the mission to be redesigned.
That said, some of the extra cost is due to scope creep, or due to changes in launch vehicle availability requiring the mission to be redesigned.