Well if you order room service once a day for 7 days it's less than $70. Disney includes soda/pop so if you drink that, you'd have to add it on. Soda pkg is about $15 - 18 per day, per person, including gratuity and depending if you can get it on sale. But you could go higher for Drinks if you wanted to. Or no soda if you don't need it. So $126 per person for a week, times 4 people, plus the room service... $574. I'm going to assume that's not quite the difference in prices.
For the deposit, when you go through the process, if you want the non-refundable deposit, it will cost more for the cruise and you have to select the option. However, I think if you choose a guarantee category you don't get the option to choose the refundable deposit. On the checkout page it's under where you choose the dining time you want, and the option to include the gratuities.