The one and only time Chrétien de Troyes names Merlin is in the remark that during Arthur's day sterling had been the currency throughout Britain since Merlin's time. Malory mentions three units of currency: besant or bezant, pence, and pound.
Until the United Kingdom put its money on the decimal system, 12 pence equaled one shilling and 20 shillings equaled one pound. A guinea was one pound plus one shilling. A besant was a Byzantine gold coin which finally varied in value between the English sovereign and half-sovereign, or less. Silver besants were also struck, and were worth between a florin and a shilling. The English florin was issued by Edward II, and was worth six shillings or six and eightpence. The English sovereign was a gold coin minted from the time of Henry VII to that of Charles I, originally worth 22s.6d, but later worth only ten or eleven shillings.
As for the buying power of these coins, in IV, 25, Malory speaks of a "rich circlet of gold worth a thousand besants" as the prize at a tournament. Speaking of Arthur's body, the former Bishop of Canterbury says:
But this night ... came a number of ladies, and brought hither a dead corpse, and prayed me to bury him; and here they offered a hundred tapers, and they gave me an hundred besants.
In XXI, 8, Lancelot returns to Britain after Arthur's passing:
[H]e made a dole, and all they that would come had as much flesh, fish, wine and ale, and every man and woman had twelve pence, come who would ... And on the morn all the priests and clerks ... were there, and sang mass of Requiem; and there offered first Sir Launcelot, and he offered an hundred pound; and then the seven kings offered forty pound apiece; and also there was a thousand knights, and each of them offered a pound; and the offering dured from morn till night.
There are also these entries:
I wot well and can make it good, said Sir Ector [to Sir Lancelot, on finding him in the Joyous Isle], it hath cost my lady, the queen, twenty thousand pound the seeking of you. Madam, said Sir Launcelot ... I proffered [Elaine of Astolat], for her good love that she shewed me, a thousand pound yearly to her, and to her heirs. And so upon the morn [Elaine of Astolat] was interred richly, and Sir Launcelot offered her mass-penny.
This seems to be about the extent of Malory's concern with the particulars of money.