The cost of each semester varies and then you have the cost of text books as well, but about $1000 for this semester but it was $1100 1st sem last year. And a couple of hundred for text books, though the texts often cover a number of subjects.
Check out half way down the page
here for entry requirements, but for the Grad Cert you don't have to have a Tertiary Qual.
They also run short courses but I've not looked at them. You can find all the relevant info here on their home page
http://astronomy.swinburne.edu.au/sao/