If you are booked on the Golden from Barcelona to Istanbul they have
removed $45 per person from the Government fees -- the amount of the Turkish
Visa. They will be selling them on board ship for $45 per person.(This can be added to your shipboard account) The gov fees should have been lowered and your agent should adjust this for you. If you start in Istanbul you have to pay the visa fee at the Airport in Cash!! Your agent may not know about this yet.
When I embarked aboard MV Grand Princess in Istanbul in 1999, Princess provided the visa with my cruise documents. I don't know the whole of the current fee structure, but I do remember that it was cheaper to obtain the visa in advance from a Turkish consulate than to buy it at the airport. When did Princess change this policy? Or did your travel agent negotiate a modification to the normal arrangement on your booking that might not apply to other passengers?
If you are obtaining your own Turkish visa for any of the itineraries of MV Golden Princess that originate or terminate in Istanbul, be aware that you need a "multiple entry" visa rather than a "single entry" visa due to the port of call at Kusadasi. The Turkish "multiple entry" visa is valid for any number of entries into Turkey that occur within ninty days of the first.