$100 per person is needed if you go through Royal Caribbean, which I recommend. If you go through a travel agent you have to pay their fees and a certain amount per month towards your cruise. If you go through Royal Caribbean, there are no extra fees and you also do not have to pay a certain amount per month. They don't care how much you pay or when you pay it, as long as the cruise is paid off 60 days before travel date. I know, I've done 6 cruises (I do one every other year) and 5 have been with Royal Caribbean.
The cost to reserve (book) a cabin is usually $250 per person. Most agents, after booking the cabin(s) will allow you to send in a payment every month towards the total price of the cruise with the understanding that the total amount of the cruise must be paid in full 90 prior to the cruise date.