I think Harry's post might be somewhat misleading. Whether you pay cash or the expenses are posted directly to your ship board account, you still will need to pay the bill. So, for saving purposes, you should recognize that there will probably be a $10/day per person "tip" automatically added to your account. Drinks range from $4.95 + 15% gratutity on up. So, figure how much you plan on drinking. Soft drinks cost also (I either brought my own, or bought the soda card, about $32 for 7 days when I got mine). There are gifts/souviners, which may be purchased onboard and charged directly to you account, or purchased in port. I'd take cash for those, to avoid the unknown exchange costs of credit cards. Also, you should figure in any gambling expenses, in addition to shore excursions, whether paid for in cash (if you arrange you own) or billed to your account (if purchased through the ship - which I've never done). You onboard account must be closed out either by charging to a credit card, or by paying cash, the last day of your cruise, before you will be allowed to disembark. Haven't tried it, but don't think you can get another free cruise by not paying
My shipboard charges rarely exceed $50 + the "tip", but then I pay for all my excursions separately. I also don't drink or gamble when onboard. I do take lots of singles for tips and trinkets and cash or prepay for my excursions. I hope this helps some. If you have more specific questions, please post - I'm sure someone will be happy to help.