The night before Terabayashi was working on a model in the room that became the crime scene, but it wasn’t mentioned as being at the crime scene, right? I assume they would’ve mentioned if it was there, so… I wonder what happened to it. If Terabayashi had the key to the science lab, and he was attacked and the science lab key is now missing, that locked room is trivialized, so I’m sure a curveball is going to be thrown… It’s also possible the 公会堂 key was dropped by the murderer outside, trivializing that locked room, but that seems like the theory the police are going to argue to avoid dealing with a headache impossible crime, rather than the actual solution.
I asked a Japanese mystery discord and someone said the 万 is probably a typo. Which would be a bit odd, since it shows up twice (in the price and then the change), but deleting the 万 makes the prices make much more sense.
Now that I think about it: the price was not given as a fact, it was only stated by the shopkeeper (just like the change). Maybe it’s a game between the two?
I did a double take on the 万円 too and after looking it up it seems that it’s just a joke/funny way of saying a small amount. weblio has an entry for it and lists it as 大阪弁 even though our book takes place in Nagoya.