The deposit is $100/person. The advantage is that you don't have to pay any additional deposit until final payment, PLUS you get onboard credit for that next cruise, varying in amount based on what type of cabin you reserve and length of cruise. You can make an "open" booking, meaning you have a certain amount of time to make your decision as to what cruise you want to take, but the deposit is taken care of and you have the onboard credit locked in.
If you think you'd like to take another Princess cruise within 2-4 years, it's a great option. The deposit is also fully refundable.