You are right....Carnival uses a cashless system...you have a sail and sign card for all purchases which includes bar sales too. We usually have to think about how many days we are going to be gone and figure our travel days to and from the port for cash for on the road purchases (food, drinks, etc). Also, you would want to include cash for any purchases of food or drink in ports. Some of the shops in ports actually take credit cards, but we prefer to pay cash so we don't have to worry about the translation of their $ to American $. A few extra dollars for tipping if you have room service and if you wish to give a little extra to your steward or wait staff at the end of the cruise. The normal tips for these personnel is automatically charged to your Sail and Sign
Card at $10 per guest per day. And then there is the casino. Although, you can also use your sign and sail card there also. If you are parking in the cruise terminal, you will need cash or credit card. Length of cruise determines the fare. Our last cruise, 7 night was $84 for parking at terminal. It just really depends on how many places you visit during your cruise and on the way to and from as to how much cash you would need.
This may not really help as far as a dollar amount but this is our experience in things to consider when allocating cash to take.