In that case I would go for the cheapest 12V to 5V I can find and then, knowing that a bunch of this devices are using UART for communications, isolate the UART lines.
I have never tried it yet but I know that maxim has some application note on USB isolation. There are isolators that work bidirectionally (I2C for example) but I don’t know how fast can they get to be.
That is an interesting question @bobc.
I was rather amused at how the op made the leap from discussing net-ties and pin type to suddenly discussing ground isolation.
Isolating USB is achievable up to about 150Mb but it gets much more difficult and expensive as data rates increase and the number of data lines increase. A far more practical method is to provide the isolation elsewhere. For example the main electronics could be power by either USB power or an isolated DC-DC converter and isolate the signals connecting to the motor and extruder drivers. These signals are very low speed and isolation is easy and inexpensive.