The cost is estimated to be $AUD250 at this stage.
They need expressions of interest though. If enough are received, as I understand it, the convention has a real chance of going ahead. Already there are a few of us who have put our hands up (quite frankly, I'd camp out for tickets to this gig).
Here is a copy/paste of the post on the AFA forum, which contains the email address to send your expression of interest to. So what are you waiting for Aussies?

Please pass this on to anyone else you think may be interested in attending the convention.
Cheers.
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Wonderful news everyone! A very exciting occasion is heading our way next year. Atheist Alliance International, of which the Atheist Foundation of Australia Inc is a member, is proposing we combine with them in having a convention in Melbourne in March 2010. With guest speakers the calibre of Richard Dawkins, Phillip Adams, Catherine Deveny, and yet to be finalised notable international and national presenters, it will be a once-in-a-lifetime event not to be missed. The convention will run from Friday night 12 March until Sunday midday with the cost of registration and meals about $250 per person and less for concessions. The theme is “The Rise of Atheism.”
We are still in the early planning stages but it would be of great assistance if those who have an interest in attending the convention contact head office by phone, letter or email and let us know. An estimate of numbers is critical to the success of this venture. Just write the word “Convention” in the subject line when replying by email.
See you there!
David
Contact:
Email: info@atheistfoundation.org.au
Phone: (08) 88352269
Atheist Foundation of Australia Inc
PMB 6
Maitland SA 5573