How much is it usually cost for a Disney cruise ship?

Don't know how much the ship would cost, but you can find out the cost of a cruise by going to the Disney Cruise Lines website and looking it up. The mass-market cruise lines will run about $100 per person per night, as a ball-park figure, but Disney is the most-expensive of the mass-market lines, so it will probably be more than that. Don't know how you got the $7K figure unless you added up the cruise fare for four people, but it certainly isn't per night. It sounds more like the total fare. The Dream is the newest Disney ship and is in high demand, thus the high cost.

Sounds about right for the whole cruise. I once tried to book a balcony cabin for my family of four and it came out to be about 14 grand! Needless to say, we didn't go.

That's for "most" of the trip-- cabin, food, majority of activities. Of course there will be additional expenses. Like if you want to get off and explore a port or if you want to get a gourmet coffee drink each morning, or use internet access or eat at one of the fine restaurants.