We usually bring $1000 cash for a 7-night cruise. We always stay the night before at a nearby hotel and that eats up a little of the cash. Our ship charges usually are in the $1100 range including auto-tips, and we usually end up bringing around half the cash home with us. And we'll add about $400 or so to credit cards while ashore.
All that said - I guess I am in the $100 per person per day boat too.