The US Postal Service has the large Priority Mail flat rate "game board box" (the usual large flat rate box is too square) that will go anywhere in the world for $58 (and it will hold 2 Model Ms if need be!).
For getting a full-size keyboard to Oz, that is about the best you can do.
A small, light board, in a not-too-bulky box, might go for $40, but that is about it. I have tried to find a better way, with no success.
Some people on this board have hinted that FedEx has some sort of "semi-priority" package that will go anywhere for $40-50, but I have never seen it, and suspect that may be some private proprietary commercial deal.
Good luck!