Dunno how you define cheap, but I paid $110 recently ( cheaper if you book online ) through AirRoadDirect ( http://www.airroaddirect.com.au/ ) to bring a gearbox and bits up from geelong to sydney. Was about 60kg, and box was about 100 x 60 x 50. (was door to door service as well).
Generally, at the sizes that you are looking at, they will bill you on volume rather than weight. I'd check the site, and see how much it works out at (you can ignore weight warnings, it will still give an accurate price). Its a dream to actually have a site that will give you a price without calling or giving your life story to . . .
I found airroad to be one of the cheapest mobs, and about the only one who would deal with a private individual (most companies only want to do business work for some reason - very frustrating). If you find the price good, give them a call and work out packaging - the people I spoke to were very helpful . . .