I don't follow them, but I also find it illogical to find a trap that costs more than the rest of the dungeon put together.
First, the destruction trap's DC is wrong. It should be 20 Fort, so the monk should have made the save. So, we're dealing with an even more powerful trap, with heightenings and some kind of insane DC boosting going on. Should the PCs expect anything like that? The DM is purposely raising the DC in order to increase the lethality of an already extremely lethal trap.
Second, the whole setup is completely metagamey. It's a lever in a room. Maybe it lowers the dumbwaiter. Maybe it signals for someone in another room to come attend to whomever is staying there. There are lots of reason to have a lever in a room that have nothing to do with the secret door and have nothing to do with traps.
That's what levers are for. To do things. Nobody sticks a randomly trapped lever in a room for the sole purpose of killing curious intruders. You put the lethal trap on a place they're sure to go/touch/mess with. To expect it to be trapped is completely metagaming, and I don't plan out dungeons based on metagame views and I don't take character actions based on metagame views.