I believe from UK to Aus the overflight permits, landing fees and official bribes run to the order of £20,000 thats before you've spent a penny on flying. Also fuel is indeed the biggest problem, for inspiration read Dave Sykes account of flying there in a microlight with his wheelchair fitted to his flexwing microlight. I had tears in my eyes reading his blog daily during the flight, and bought his book and chatted to him at the LAA rally.
But do it anyway, you only regret the things you don't do, rarely regret those you do.
I am saving up for a round the world flight in 2015, but will be using JetA to get around the fuel problems.