For things in Earth orbit (LEO all the way up to GEO) we do have some fairly worked out buses that can keep commercial missions done on budget within the fairly

For things in Earth orbit (LEO all the way up to GEO) we do have some fairly worked out buses that can keep commercial missions done on budget within the fairly well developed parameters of commercial up to GEO. Have, honestly, since the 1980's. Multiple companies from multiple countries have demonstrated this, there is a competitive market for commsats and earth observing satellites (the only two markets where business cases really close). If you are doing science from LEO you can probably re-use a lot of components from those markets. 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.