My business partner and I are flying to Xi'an from Brisbane for maybe 2 days to sign a contract.

John, I advise you to fly from Brisbane to Xi'an via Hong Kong and then fly back to Australia, so that you don't need the entry permit.

If you just want to fly into Xi'an and then back to Australia, you need to apply for an M type in advance fom the CVASC office in Brisbane. The cost is 109.5 Australian dollars.