There was a snarky comment about how he was being sarcastic on the tip of his tongue, but then what Kurt actually says sinks in.

"It's seriously still just a quarter?"

Not that he ever had to deal with the tooth fairy, so he had no way of knowing if it was ever a quarter to begin with. But it was precisely because it was something he never experienced that he was interested in it... it was just one of the many little things about childhood that he just didn't get.