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What we tend to do is have a look at the prices on the ship before going ashore so we know what to compare like with like, but in some cases it is cheaper on the ship.
Best tip we got was check the prices at home first, or you will not know if the ship or the shore prices are the best,and check if the ship does a price match you would need some prove such as a leaflet but some will do it Best of luck .
Whatever part of the World you plan to journey, keep an eye on the High Street Currency Exchange Market ( some shops offer Senior Citizen deals on special days.) When the currency you need is high - buy an amount for future Cruising - don't wait until you leave British shores as you may not get the best deal.