Well, a coin could definitely be classed as a "nonmagical trinket", so yes, you could make a coin with Prestidigitation, but there are a couple of things to consider: Firstly, this trinket is going to last "until the end of your next turn.", a whopping 6 seconds, so it's unlikely that Prestidigitation-created coins are going to be much good for trade! Secondly, the cantrip has both verbal and somatic components, meaning if you're planning on pulling a fast one with the shopkeep, it's going to be incredibly obvious you're casting a spell.