The party is level 5, way too weak to go all the way down - they are only going there in search of an item that belonged to one of the previous parties that went there and never returned. - the first underground level is still pretty dangerous but much has already been disabled by previous groups - it should scare them but they can back down at any moment so all should survive.
Be wary of this kind of thought. If there is something I've learned from my DMing experience is that PCs very rarely do what you expect, especially if it fall under reason or common sense. This is a game and some players are simply content to send their characters onward to trouble since that's pretty much the game simplified right there. Some other expects you to build challenges and encounter they can actually have a chance of overcoming so... I guess what I'm trying to say is expect the unexpected. If you really want your group to fall back without casualties then make it clear OOC that they can't do it, or give them some very strong incentive IC not to dwelve too deep.
About the trap itself, I'm very fond of the treasure illusion too. I use it often. The last time I did was for a non lethal challenge. The lethal part of that treasure was to be able to overcome the traps on the vault door (there was three of them and the rogue didn't manage to overcome the last trap and fell down with 1 wisdom left. Good times.
) Once that was done, behind the door inside a small chamber was a single mirror engraved in runes. Someone managing to decypher the runes understood that by pronouncing the power word while touching the reflecting surface (the word was provided) one would be transported inside the real treasure chamber. It was made clear in the explanation that once someone got there and came back, return to the treasure chamber was impossible.
So, once inside the treasure was of course an illusion which disapeared as soon as someone got close enough. At that moment all light already provided inside the chamber disapeared and any light source used by the PCs were dimmed to one square away. They were almost blind. Furthermore, when they tried to explore the chamber they realized that it was looping back to itself. After much trials and errors, they also realized that there was a second mirror on the wall facing the mirror they entered when they walked in a straight line away from it. The power word didn't work on this one though, as one needed to move backward from the first mirror down to the second and then use the power word to finally get inside the real treasure chamber.
To my surprise, the PCs found everything out with time and they got out with a huge increase in magic items, weapons and armor.