The auspost website is hard to navigate (i prefer to just walk into a shop and buy one, but they shut the shop near my work), but I found the
calculator that gives a 500g airmail padded envelope as $14.10 (which is actually only US$12.40, dollar must have dropped again).
It's the US post that's expensive, I rarely get anything bigger than a CD for less than $30US, minimum anything from adorama/B+H is about $60 so I make it worth my while and buy lots at a time.
Meanwhile, back to the board, I measured mine and it's a #0 size, so no good for you anyway (unless you've got a big metal drill bit).