We design this stuff with "cold equations". These missions always go through an initial "Pre-Phase A Study" where we do the math and see if the proposed mission is practical. Really bizarre things are proposed that never see the light of day because they get clobbered at Pre-Phase A, e.g. a Pluto lander, Venus sample return, etc. If it gets through Pre-Phase A then a formal proposal is written and the mission competes against other mission proposals for funding. Most successful Pre-Phase A proposals are not funded and simply disappear. The funding proposal phase is where the whole issue boils down to bang-per-buck. That's where the Europa mission fails to pan out. It requires lots of bucks with almost no "bang".

Gary