There is no good way to answer your question without at least some more info. First, as you asking how much people spend or how much cash they bring (a lot of people tie their account to a credit or debit card and bring/spend almost nothing in cash while others prefer to spend cash). Or are you asking about a cruise budget over and above the ticket to get you onto the ship?
Things you can be spending money on:
Transportation from airport to port and back--$35 per person each way at PC, other prices at other ports, private alternatives available
Suggested tips--$12 per guest per night of cruise...so $84 per guest on a 7 night cruise. That's $168 for 2 of you
Beyond that, it gets totally subjective based on what you WANT to spend/do
Adult dining--$20 per person + tip
Photos-$0 to $400 (CD + prints)
Alcohol, specialty drinks, specialty coffees
Spa--$16 one day rainforest pass to $500 + tip or more for spa villa for 2
Excursions--$6 for float on Castaway to $400 for "dolphin trainer for a day"
Bingo--this one adds up fast
shopping--nuff said
This may sound crazy, but we've been on cruises where we spent basically nothing beyond tips and we've done it where we spent a LOT of money--it doesn't make that much difference in the cruise to us. Yes, there were excursions that I'm really glad we did. But spa? Yeah, it feels good, but then it is over and the big money is spent. And I can get it at home for half the cost.