Our TA suggested between £50 and £100 per couple per port. It wasn't far off.
I found if you were doing ship's excursions you spent less as you didn't need money for travelling about in port.
Using this we worked out before going which ports we were likely to do excursions before going and took the £50 for those. It worked the last 2 cruises and we even came back with money the last 2 cruises.
1968 S.S. Nevasa Baltic
1970 S.S. Nevasa Western Mediterranean
2008 Island escape Western Mediterranean
2009 Legend of the Seas Eastern Med and Aegean
2009 Island Escape Canaries
2010 Thomson Dream Western Mediterranean
2011 Thomson Spirit Red Sea Magic
2011 Star Princess Adriatic and Aegean
2012 Thomson Majesty Adriatic
2013 Thomson Dream Western Med