Onboard bookings have a 10% discount on the cost of the cruise, as well as Onboard credit ($100 for cruises less than 7 nights; $200 for 7 nights and longer). Also, 7 night and longer cruises have a 10% deposit (regular deposit 20%).
As prices do rise as the space onboard fills, I'd suggest you go ahead and book the cruise now (locking in today's price). Once onboard, you can "reshop" the cruise. Since the discount is taken on the prevailing price, as long as the price hasn't risen enough to negate the savings, you can cancel and rebook. If the price has gone up too much to cover the savings, you've gotten the best price you can.