If you built even 15 Europa Clippers the cost per-item would come down enormously (because all of those people's work could be re-used), but since the 1970's NASA has not had the budget for multiple probes per missions. So every mission is bespoke, and has to be done again completely from scratch.
1: The engine was normally used for circularizing the orbit of a geosynch comm satellite, so within a few hours of flight. For doing a Mars Insertion burn it needed to sit fueled for months in outer space, which was not appropriately tested, and probably the fuel tank exploded in flight because of that.
Each mission has unique requirements, but since payload mass costs are coming down, ISTM it should be possible to create a standard buss that meets most requirements most of the rime, even if it's heavier than a bespoke effort for any one mission.
https://en.wikipedia.org/wiki/Comparison_of_satellite_buses
For example, the SSL 1300 apparently has hosted 118 satellites so far:
https://en.wikipedia.org/wiki/SSL_1300
Though maybe the distinction between "satellite" and "spacecraft" bears importance here.
Is there room here for making things more reusable? For example, instead of creating one big satellite with tens of instruments, how about they create 10 satellites with one instrument each? or would that still be too bespoke to lower the cost per item?
The whole strategies of exploration haven’t shifted yet to this new paradigm. Hopefully NASA starts making smaller probes and launching them far more often.
For exploration missions and anything in deep space (basically, beyond Lunar Orbit) people have kicked around ideas for common buses, there have been plenty of proposals, but no one seems to have enough value in them to be the third or fourth user of one- everyone has found it better to start from scratch than use someone else's bus design. It is possible if there was a sustained, focused effort at one kind of project, say, something where Mars orbiter launches were guaranteed every 26 months for more than a decade, that the investment in a common bus might pay off. But as long as we are bouncing between Mars, Jupiter, Pluto/KBO's, E-S L2, and inside Mercury's orbit, it just isn't actually reusable.
Just as one point, until the past few years everything in the outer planets had to be RTG powered, which requires a totally different design than something solar. It was only with Juno (and now the Europa Clipper) that solar has been demonstrated for outer planets at all, and it is still not exactly a design you'd have off the shelf, nor would the power design you'd want for outer planets solar be at all similar to the design you'd want for inner planets solar. The same is true for comms, for thermal management, for rad-hardening, etc.
It is possible for a swarm of small satellites to fill niches in space exploration. Closely studying Europa isn't really one of them with today's technology.
Easier said than done, of course.
There was some talk about using ISS as a base for on-orbit assembly but the orbit (half-way between best orbit from KSC and Baikonur) isn't great for that and it turns out that constant docking and un-docking ruins scientific experiments requiring microgravity, so ISS really isn't a great base for assembly. Ideally, if you want to start on orbit assembly you'd have another station in the right orbit for KSC which isn't doing any microgravity research, but now we're talking about massive up front investment to save money on research satellites, is NASA ever going to get the scale of research budget for that savings to be worth it?
If something like Space Based Solar Power ever become a thing then such an on orbit assembly station would make sense, but the case for assembly for science missions really only closes if you have the station already for something else.
I would also count the Apollo missions. They launched on a single rocket, but the docking between the CSM and Lunar Module was for all intents and purposes equally difficult to assembly on orbit.
There are multiple commercial companies planning to assemble stations over the next few years. This in addition to the on-orbit refueling that SpaceX will be doing should hopefully enable a new generation of larger, assembled interplanetary probes.
But that is closer to what I meant, because they had to worry about separate launches that might fail (GVI, GIX) and find and dock with something you need to use orbital mechanics to approach (basically anything under 30m distance you can just eyeball and fly, but anything over that distance requires the full set of orbital calculations).