Hi Bev
I did this trip 2 years ago, and have also been to Australia on my own a few times, both before and after the JY trip)
Australia is a great country, (usually) has amazing weather, and it's people are very, very friendly. The one big drawback, is that it has a very strong currency, and as a result, is very expensive. If you eat in the centre of the big cities, expect to pay at least double what you would in the UK for a similar standard meal, and probably nearer to treble in Sydney/Melbourne.
(It's possible to get good quality food cheaper, but you have to be willing to get away from the main tourist areas, personally I think this makes a more interesting trip, but it's up to you how much you're willing to explore. In my eperience most people on a JY holiday will stick to the areas near to the hotel, but there will always be a few who are happy to explore)
If this is the "holiday of a lifetime" then I guess you will want to do all of the optional excursions, and this is fine, but will get pretty expensive.
There are also things like the bridge climb in Sydney which may appeal to you, but isn't cheap, I think it's around £130, but you need to check their website. (To me it's a must do experience - you don't need to be that confident with heights, just OK on an ordinary ladder).
If you look back you'll find loads of threads concerning this trip, which may be of help to you. Also, I'm sure Ben will supply the exact costs of the excursions if you ask him directly in the "Ask the editor" section
Have a great holiday.
Stuart