Depending on what your freight charges are from Australia, that is actually not a bad deal for 20 sheets of Depron compared to getting it in North America. A box of 20 sheets 700 mm x 1000 mm is $135 USD with a slight curvature. http://www.rcfoam.com/depron-and-epp-foam-suppliers/6mm-depron/6mm-white-depron---case-p-343.html
To get it shipped to me where I live in Canada would be another $47 USD, so just over $9 USD per sheet. I have actually been building a lot of planes using a combination of Depron and Adams readi board lately, using the readi board for wings, fuselages and vertical stabs if the design allows and the Depron for nacelles to give good longitudinal strength and elevons to keep them strong without having to add any reinforcement or extra weight to the back of the plane. Using the thinner readi board for the wing and fuselage also cuts down on the drag profile making my planes faster and/or more efficient through the air.
I shot this video awhile back discussing my current build philosophy of combining different foams to save money and to take advantage of the best properties of each while stretching my supply of Depron as far as I can
It takes awhile to figure out what works best depending on what you can get, what you can afford and what you like to build with I have found building park jets for almost six years now
And once in awhile the postman wants extra money for duty, not always but about 1/3 of the time. But I do like my Depron.