I think you quoted an 'estimated' cost for a Europa orbiter, not for a Europa lander/ice-penatrator] mission.

There will also be cost overruns for any ambitious
Europa-lander
Nasty cost overruns!
Not cost overruns measured in percentages
but in magnitudes.
The ISS cost overruns will be small potatoes
compared to what will be needed for a Europa-lander/life-search mission.
[Yes!...That's my speculative opinion!....I hope that
you won't be dismissive; NASA's recent track record does supports my opinion]

Also, the VASIMR propulsion system R&D and Earth-orbit assembly
costs will represent only a slice of the total mission R&D, assembly,
launch, and mission operation budget.
[I'm using the Deep Space 1 R&D, assembly, launch and operations budget as an example for how the 'pie will be divided'.]

The VASIMR propulsion system will be enormous
by itself; easily larger than today's space shuttle.
[again an opinion, but one obtained from viewing
CAD illustrations made by NASA designers of the Project JUNO "Europa- orbiter" stack.]

Yes, you can use a nuclear-powered VASIMR
propulsion system to send a large payload to Jupiter
surprisingly quickly and then use the system to park it in orbit around that planet.

And the VASIMR can even provide viable propulsion
to brake the very-large robotic payload into Europan
orbit....But that's it!

The VASIMR propulsion won't help the life-searching
Europa-lander go from orbit down to its icy surface.
You'll need hypergolic fuel for the Europa lander.
A lander that will be "huge" if it is to equipped
with an ice-penetrating probe with a tether
1,000's of meters long.
The delta-vee from Europa orbit down to its surface
is not far from that required to land a spacecraft on our Moon.

As a result, the lander will probably be as large or larger than
the Project-Apollo LEM when you add the surface rover(s), all the radiation shielding, the tether-winch, a submersible probe,
redundant mechanical, communication and power systems, etc...

I think 100 billion dollars [to be spent over a 10-15 year period] for such an ambitious unmanned life-search mission to Europa's surface would be a reasonable but crude estimate.
And all it would find is a sterile, highly-acidic subsurface 'ocean'
on Europa. [that 'opinion' is based on GALILEO's data]

I stand by my claim that a mission to Enceladus' surface instead will
be more profitable for a life-search mission, and much less
expensive.