Hi Clare
It's true that it's expensive but I wouldn't say that it's any more so than, say Italy. The great thing about this trip is that your have a lot of your meals included in the price (usually you only get breakfasts) and that really saves you a lot. The food - by the way - is amazing.
Another tip is that the hotel does a 'Happy hour' (actually two hours) every evening and, while I'm tee total so it didn't really help me, most of my fellow JYs were able to 'stock up' on beers etc in the evenings.
The best tip however, is to make sure that you have all your till receipts validated and keep them safe, because at the airport you can claim back all of the VAT you have been charged. It's a really simple process and you can either take cash (if you have time to wait while it is calculated) or they can put the money back on your credit card which takes about ten days.
Other than optional entry fees, for example the clock tower at the cathedral is a must even if you don't like heights, there's really only souvenirs, snacks and tips for your guide to think about.
Have a wonderful time - I'd love to go back one day
All the best
Mandy