The tipping is $10 per day per person travelling. 8 of you would be $80 per day..... and that includes tipping everyone on the ship who serves you, aside from bar service.
This may sound like alot because there are 8 of you, but especially with 4 in a cabin, and lots of teens, the staff you are tipping will likely earn every penny of it
You can remove the automatic tipping by going to the Guest Relations desk, and asking them to be removed.
As far as extra costs onboard.. aside from drinks, there's shore excursions (if you do any), shopping, souvenirs, gambling, purchasing photos if you choose. So, there's no extra charges unless you chose to avail yourself of services or goods that cost extra.