We were in the same situation as you and expected the full cruise price to be refunded. As I was not sure how much the hotel accommodation was I didn't feel in a position to argue, also wondered if they had deducted the price of the flight. Have submitted a claim for extra expenses incurred but as yet have heard nothing.
Where on earth you came up with the calculations that the island star is 4 times the price of a holiday in Mallorca I will never know.
I live in Mallorca (am from London, here 18 years) and was on the island Star cruise two weeks before you. It was good value for money and we had a great time.
As far as your payment for the cruise being returned then I cannot help you with that., however, I can reassure you that there is no way on this earth you could holiday here for a quarter of your fare unless you had booked a suite with terrace and spend day and night in the paying restaurante and drinking constantly.
Have you seen this link? http://www.islandcruises.com/press/release/17