As others say IT Square, you can catch the monorail, make sure someone has registered you on a TM30. so that immigration can see you on the system. 1900bht in cash.
The thing is now, the ticket prices are like fuel prices in Victoria, up to 20% difference over a week, plus Thai caters for the Indian and Israeli Market, who transit through bkk, so they charge accordingly, Aussies to Bkk are only a small part of their business. I now look at all the other airlines and go via either Singapore, KL, Vietnam,Taiwan or even China, comparable prices and something different along the way. I am waiting to see what the new Turkish airline will do to prices as it increases seat to Europe, which will create space on transit flights through Bkk.
My concerns as farang married for 50+ years, make sure you have registered mortgages going forward. Always have a 3rd party interpreter at any business dealings, make sure all relevant permits are in place, right down to the local village headman. Check for outstanding loans, both registered and not.