This flashback takes place in France (in the Arthurian kingdom of Gannes, which is more or less a stand-in for Gaul*) so we’re dealing with French money here. A denier is more or less a penny. Not very valuable.
*Arthurian places are very weird and nebulous, and it’s hard to pin down any real borders. Gaul is just somewhere in France, likely a neighboring kingdom to Benoïc. And Benoïc just might be somewhere around Normandy, or the Loire valley if my memory serves right. But who really knows.