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The first time we got a free upgrade to all inclusive I worked out whether it would have been worth paying for it (I'm an accountant!)and the answer was that we would have just about broken even on it.
We tend to drink quite a lot on holiday, don't forget soft drinks are included too (but NOT bottled water), but unless you drink very little then I think it's worth it just so you don't have to think about what you're spending.
Depends how much you drink! the prices on board are about pub prices I think...£2.25 for a glass of wine or cocktail of the day..don't know about beer...can of coke or sprite £1.00...but the all inclusive is about £150 for the week I think so if you drink alot it may be worth it.. but remember you probably won't be onboard during the day so it would just be the evenings. We were lucky and had the free deal as we booked early!! Can't help any more than that I'm afraid.
Personally, it is worth taking the all inclusive deal, I agree with Guy from Halifax that it takes away the worry about what you are spending because if you do the shore excursions too, this will add up to quite a lot of money in the long term.