My first thought was also that it was Latin and a possible gag by Jonson. After investigation however, I find the phrase comes from Jonson’s play, Bartholomew Fair and, if we accept the evidence at this site, it was inscribed at the request of one Jack Young.
What does it mean? My WAG is that Mr. Young chose it for several reasons. First, it was a quote from Ben himself. Second, “rare” can mean uncommon as well as excellent or extraordinary, all of which apply to Ben. And thirdly, if the stonecutter charged by the letter, it means Mr. Young was careful with his money.