The silver penny is an ancient silver coin.
"A witchman, mumbled the woman. Called by some a witcher. To summon him is most dangerous, albeit one must; for when against the monster and the vermin there be no aid, the witchman can contrive. But careful one must be—
Eeee... But careful one must be to touch not the witchman, for thus the mange can one acquire. And lasses do from him hide away, for lustful the witchman is above all measure—
—though the witchman greatly covetous and greedy for gold be, mumbled the old woman, half-closing her eyes, giveth ye not such a one more than: for a drowner, one silver penny or three halves; for a werecat, silver pennies two; for a plumard, silver pennies—"
- pg. 180, The Last Wish (UK edition)