To set up your shipboard account with cash, I would figure in the automatic tip of $10.00/pp/day...so for a 7 night cruise thats an automatic 140.00 charged to your account. Most of the photos taken on board range from $7.50 - $20.00 depending on the size. Each day there will be a drink of the day offered, a fruity type alcoholic drink, usually around $6.50/each if you keep the souvineer glass, or around $3.50 without the glass..plus an automatic 15% gratuity tapped on. Beer runs around $3.50-5.00 depending on brand you drink, they usually have a "bucket" special on NCL its a helmet of beer, you pay a deposit of $5.00 for the helmet, and then you get 6 beers for around $19.00 - $22.00. Keep the bucket all week and return to any bar the last night and you will get the $5.00 refunded back to your account. If you purchase anything at the gift shop it is charged to your onboard account. Also, if you book any shore excursions on the ship they will be charged. Bingo cards can either be charged to your account or some ships let you pay cash for the cards. So as far as how much to take, it depends on if you are heavy drinkers, and how much you plan on spending on souviners.
I pesonally have had my onboard account reach $1700.00 (we spent money at the art auction) and partied the whole week for a 7 night cruise to as little as $500.00 for the week, just depends on you.
To set the account up onboard with cash most cruise lines require at least $350.00 put up front, but if you go over that amount, you can be assured that they will notify you to come to the service desk to add more money. If you place a large amount on the account and don't spend all of it, the last morning of the cruise you will find a check for the amount not used under your door.
My advise.....2 people --7 night cruise plan on around $800.00-$1000.00.