Hi, I think the problem is you are underestimating how much of pain in the ass it is to send an engine half way around the world. Customs clearance, quarantine inspection, duty declaration, sea freight organisation, packaging and road transport to the wharf etc etc. It's not so bad when you are sending an engine with some value, but is it the same amount of work for a $400 engine as it is for a $6,000 engine. In fact it is even more work because a freshly built engine will pass inspection easily, a dirty, oily, old, used engine is not so easy.
Plus I guarantee you that you will not be able to rebuild an RB30 (to whatever spec) cheaper in the US than what we can downunder. It's false economy, but your choice nonetheless.