I booked mine last summer through Expedia or priceline and when I saw a special rate much lower than the one I had on the other site (can't remember which one I booked vs cancelled) I cancelled and rebooked at the lower rate. It was about $200 as I recall.
I would say save your money. I did not think atlantis was worth the time. We did not enjoy it at all. The food was so overpriced (I saw your other posts about food) that we left before lunch and went back to the boat. Just my opinion but I wouldn't do it again. I took a nice picture of the tower suite there and can say we were there, but it was a waste of time and money for us. It was me, my mom and my 6yo nephew. None of us are very brave when it comes to rides, but my nephew didn't even like the rapids lazy river. The lines were long, there were no chairs or lockers available, and it just wasn't for us. I'd say stay on the boat. I know that you weren't looking for my opinion on atlantis and I know alot of others enjoy the place.
I'd say keep watching the rates, b/c they do offer specials and stuff. It will come down.