I work for NASA in a planetary science organization, so let me say this: cost accounting of space missions is unique to the way NASA/ESA operates. Much of that cost is tied up in things like paying researchers and graduate students to do studies during the decades of time between missions, obscene university overheads, re-inventing the wheel with a new spacecraft design for every mission, prime contractors who attempt to maximize the number of subcontractors and resulting complexity because that awards them points in the selection committee, etc.

Private industry could do it way, way cheaper. SpaceX could probably build and launch 45 Cassini probes for what cost to do Cassini–Huygens.

Let's see how things go, thus far SpaceX has proved they can get stuff into orbit but they aren't super-cheap yet.

Private industry can do a lot of things but a Cassini probe mission for $71 million all in seems unlikely in the near future.

NASA has a contract with SpaceX for 12 supply missions at $1.6 billion while NASA says a shuttle launch was about $450 million (but that figure probably doesn't include all the costs).

So SpaceX at this stage is at about 25-33% the cost of a space shuttle mission, but that's still a long way until we get to "45 Cassini probes" for $3.2 billion.