We took the Atlantis excursion in Sept 02. I think it was $33 per person. It's a tour of the hotel, casino and the dig. It was pretty interesting. After the tour, which takes 1+ hours, you're free to stay at the casino, eat or explore some more.
There are some areas that are just for hotel guests. The tour guide will tell you that during the tour.
It was worth it for a one time thing.