Every king eventually brings out their own money. The Pendragon crest is naturally on one side, but the King's face normally goes on the other.
And Arthur does that, or intends to. But when he talks to the guy who is meant to get his profile, Arthur can't help but ask what kind of other symbols he could use. At first, he asks if he could put other people on it, then he figures that it would probably be too controversial if he put a face on the coin that isn't his own.
Then he asks if it could be a bird, like his mother's sigil. Originally, he wants to honor her, but then Merlin walks in to gather his clothes and it suddenly strikes him that Merlin has done so much for him, and never gotten any reward.
And Arthur doesn't mean his servant work, Merlin is rubbish at that. But what about his council? His advice? The fact that Merlin always speaks his mind, yet always has Arthur's and Camelot's best interest at heart. The fact that Merlin always goes out of his way to fight alongside Arthur, despite not being a knight.
In an emotional outburst, (after Merlin left again) Arthur orders the coin guy to make the other side of his coin a Merlin.
Right after that, he realizes what he's done and drowns his embarrassment in wine.
A couple weeks later, the money is created and put out into the kingdom. And everyone immediately connects the dots. The symbol is only on the golden one.
The day Merlin finds out is a normal day at first. Only that everyone is staring holes into his head and whispers about him. Merlin has been send to get Arthur's new shirts and hands the tailor a golden coin. He hands it to him, then stops and stares at the crest.
Merlin: Arthur???? What the fuck is this?
Arthur: *sweats* your salary?
