Princess often has sales that only require a $100 deposit, the same as your FCD. You don't get the OBC though unless it happens to be included with the sale.
You could book your Fall cruise this year with the $100 deposit and wait until your cruise, then book your next Fall's cruise while onboard under the 3 for Free sale that will likely happen this Fall. Then you could get free grats, obc and a minimal upgrade.
Then while you're onboard, buy a couple more fcd's and stick them in your back pocket.