daughter of Plutus
Plutus is the Roman god of wealth; the wealthy heiress Miss Dunstable, whom the Greshams desire Frank to court and marry, is referred to as a daughter of Plutus. [JM 2005]
this bird, so rare in the land
Referring to Mr. Moffat, who is rare in that he is both young and precociously calculating. This phrase recalls a line from a satire by Juvenal: rara avis in terris nigroque simillima cycno. “A rare bird on earth, most similar to a black swan,” spoken of a hypothetical ideal wife. [JM 2005]
Sources: Juvenal, Satire 6.165.
quid pro quo
Latin, “something for something,” one thing in exchange for another. Mr. Moffat is speaking to Miss Dunstable of how the aristocracy expects something in return for their company. In this case, they expect low-born but rich people to marry poor aristocrats and impart to them their wealth in exchange for induction into high society. [JM 2005]