I'd say that you need at least that much after you've paid for the flights over mate. Cairns is tropical so it's hot and sunny pretty much all year except for the cyclone season but I don't think Cairns gets hit that hard. I only spent a couple of weeks there so there'll be other posters who can give you better info, all my time was spent in Sydney. There's a good way to travel the country that I heard about but never tried so you'll have to google it if you fancy the sound of it. basically you deliver cars or campervans for car-hire companies. So for a couple of $ (you only pay for the petrol), you get to travel the country between A+B. You have a set amount of time to do it depending on how big the distances are, but it's a great way to see the country and if you take some mates, you can share the petrol as well! well worth looking in to!