Over loving expensive food (seriously €4 for a croque monsieur. Thats loving bread wih ham and cheese. Bruh)
Did you buy that on the train, or at the convention?
There's always a mark-up at either really.
Also, €4 doesn't sound too bad really. That's only £3.
Although I don't know how big yours was, or its quality.
According to some random website, you can expect one in Paris (although it is Paris) to cost you about €6.