Cost depends on when (what time of year) you go. We went back in Sept and it was $150 for a day room. but I have heard of day rooms being as much as 250.....You can't get a day room ahead of time..but only on the day of the visit...sort of like first come first serve. I have heard of people calling Atlantis from the ship when they doc and getting it that way...we just went to Atlantis and went into the lobby and asked. Or you could just book a room ahead of time. As a hotel guest you have acess to anything and everything Atlantis has to offer...including the water park/slides, pools, private beach, the Atlantis aquarium aka the dig, and water toys (ie paddle boats, paddle bikes, floats...) By getting a day room you have acess to a hotel room durring your visit..same as if you booked a regular hotel room...with the exception that you have to be out of the day room by 6pm. If you just go to Atlantis on your own for the day with out getting any room...you can walk around inside all you want, do some shopping in the shopping area, play in the casino...and there is a public beach right behind Atlantis. You can also pay admission to "The Dig" and see their aquarium....which is a nice walk around a portion of the Atlantis grounds. There is really more to do there than you could possable do all in one day. They don't do any per person rates..but by booking a hotel room you are able to have 4 people in your room...and recieve 4 braclets to get acess to everything. The day rooms allow you acess and a braclets for 5. You are not able to wander around the grounds ie the pools, lagoons, slides or beach with out a braclet...and yes they do monitor it.
Oh...and one other thing they change the braclet colors every day so they know who is suppose to or can be there that day....not that that really matters.
You can also try the Comfort Suites...which is near by Atlantis. By booking a room there you can also have acess to all Atlantis has to offer. Atlantis is huge....so the walk from the beach or slides back to a room at Comfort suites may be a ways...(i'm not sure)...it may also depend on the age and move-ablity of your crew.
One thing to keep in mind...the food there wasn't the best...and it was muy spendy...mucho....$$$$. We spent $35 on two kids chicken nugget meals and 4 (6-8oz) bottled waters. So you may want to look into packing a lunch from the ship...and for sure bring drinks of some sort.
Over all we had a blast...and were all glad we were able to do it.