I checked by googling around for quite some time: The info itself is around everywhere - I saw it in English, in German, and in French. I saw it in books, with the help of Google Books search.
But nowhere any source for it, nor any indication which Latin word could be behind it.
The only interesting tidbit that I could come up with: The oldest book with the "priceless" meaning in it that I found is from 1967:
https://books.google.ch/books?id=94psAAAAMAAJ
So this certainly predates the usual baby names websites copy-and-paste nonsense that is often the source of questionable meanings.