Start to finalize your plans by March 2007. I am booked on the Freedom OTS (of the Seas) in June 2007. I booked the cabin I wanted as soon as it became available (May 6, 2006). The cabin I wanted was the Royal Family Suite. You need at least five, and it can sleep up to eight. The cabin is 588 sq. ft. with over a 200 sq. ft. balcony. The cabin has two bedrooms and two bathrooms. The cabin is about the size of three standard balcony cabins. There are four on the Freedom OTS and the Liberty OTS, and the four cabins sold out within the first hour. At any rate, expect to be able to book cruises after April 2008 around April/May 2007. You can book early 2008 cruises now.
Price depends on time of the year. When school is out prices are high. For the Royal Family Suite for the five, based on what we are paying, it would be about $11,000. There may be other cabins that sleep five. There are family ocean view cabins, family inside cabins and family promenade view cabins, all of which would cost less than the Royal Family Suite. Based on recent research I just did for a family of four (for a late June 2007 cruise), you are looking at $1500 to $2000 each for the first four (since no standard cabins sleeps five, you will need two cabins - which is why a family cabin is your best bet) and $1000 for the fifth person.
You might be able to save some money by booking during the school year, or by booking on the Navigator OTS or Mariner OTS (the two newest Voyager class ships). The Voyager class ships are about 15% smaller than the Freedom class ships, and offer almost as many activities.
No matter which ship you choose, or when you choose to cruise, the earlier you book the better. If prices go up, you get to keep the lower price. If prices go down, you can ask for the lower price. Also, by booking early you can get your first choice of cabins.