Depends on where you're going
you have to remember your abort modes,
and you need to think about sharing the cost among other nations as well
of course, since the cost in any case is high, where "you're" going depends on convincing a large number of people to finance the trip, so the question is better put as "where we're going" - and the answer to that is "why are we going?"
http://www.geocities.com/epgrondine
ep