Many companies, including Charles when they were still in the business, and now Hubbell/Acme have two offerings.
At the heart of both offerings is a transformer with connection points that lets you wire it for either 240V or 208V incoming voltage. Such a transformer will cost in the $2000 to $4000 range. Here is one available from Fisheries for $2300
Hubbell HBL50AITW | Fisheries Supply I see it from other's for as little as $1700.
Because there are different connection points depending on the 208 vs 240V input, you need to wire the transformer with a switch that let's you select which arrangement to use. It will need to be a 50A switch, so might cost another $100 or more.
The fancy versions that cost $5000-$10000 use the exact same transformer, but add circuitry and switches to automatically detect the shore voltage and switch the wiring accordingly. So it's an automated switch, not a manual switch, and you pay a lot for it. The Charles IsoBoost was like this back when. And Hubbell offers a similar product, as do others from the references in this thread. When you see expensive units, this is likely what you are seeing.