Heres the short version, since my fiance and I are getting married in Oct on the Magic.
1st. You need to reserve your space at least 1 year in advance. There are 3 spots available to be married on the ships. 2 on the ships at different times, usually deck 7 aft, unless there is inclement weather, in which case the ceremony is moved indoors. The 3rd is on Cast Away Cay.(Id imagine that the Dream and Fantasy are the same)
2nd Current pricing is 2500 for the onboard, and 3500 for the beach.
3rd By law you will be legally married,(sign the marriage certificate) prior to the ship leaving port. The ceremony is just that a ceremony as the ship captains cannot perform a legal marriage at sea. From what I understand it has to do with what country has legal jurisdiction.
Anything else, feel free to ask.