The only $ amount involved in using points to cruise is the $75 fee per reservation. Note that it is not per person, but is per reservation. Just be sure that you have "all your ducks in a row" before you book, because if you change anything, you will pay another $75 fee.
Many people advise against using points to cruise because you can "rent" out your points for $10 per point. You can usually cruise for far less than that amount....