Budget for wifi, laundry, drinks and tips, plus any activities you pre-book like kayaking or camping. Beyond that the fare is the fare, and expedition ships don't do the end-of-cruise surprises that mainstream lines are known for. There are no port charges, no taxes bolted on at disembarkation and no overweight luggage fees.
Tips are the item people most often forget. They aren't required but they are expected, and some ships add them automatically unless you opt out. Amounts vary by operator, so ask us for the norm on your specific ship rather than working from a general figure.
One thing to check separately: entry or reciprocity fees for your nationality on your specific route have come up before and could again. That depends on your passport and where you're transiting, so confirm it for your own circumstances rather than assuming.