It’s not that expensive to do an AOC from scratch, just the time it takes to either do the manual suit yourself or get one of the many companies that do it for a living to do it.
Get the appropriate people in place and submit it to CASA, then wait. If you’ve got all your ducks in a row it should go through without too much back and forth which is where the time wasting is. Otherwise it’s up to CASA and how many people they put on it.
CASA give you a cost estimate when you submit the package.