Cost will depend on what you specifically want to do. If you look at the tours offered for your cruise by the cruise line you'll get an idea of the cost and things to do. The prices for cruise line tours are typically higher than arranging private tours on your own. However, if it is your first cruise and you are going to unfamiliar places you might prefer the ease of booking with the cruise line. This also means that if your tour gets delayed, the ship will wait for you. If you do private tours you bear the responsibility of getting back in time. In those instances the ship will not be waiting for you.
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a look for some insights on Marseille. Member Hank has significant expertise when it comes to European ports.