"Venomous Tentacula seeds," said George. "We need them for the Skiving Snackboxes but they're a Class C Non-Tradeable Substance so we've been having a bit of trouble getting hold of them."
"Ten Galleons the lot, then, Dung?" said Fred.
"Wiv all the trouble I went to to get 'em?" said Mundungus, his saggy, bloodshot eyes stretching even wider. "I'm sorry, lads, but I'm not taking a Knut under twenty."
"Dung likes his little joke," Fred said to Harry.
Harry Potter and the Order of the Pheonix
I think "I'm not taking a Knut under twenty" here means I would only accept your money if you paid me above twenty Galleons. But I'm not sure if my understanding is correct. Does the sentence content any idiom?