With all the major telcos purchasing a phone plan + plus the phone has in almost every case become more expensive than purchasing the same plan without phone, and then purchasing that phone in retail (grey import or not).
I compel thee, fair maidens of OzBargain, to show me a telco phone plan that is a cheaper (give or take with contract flexibility), than buying a BYO plan and phone separate.
Well, that's going to affect your calculations. Be fair and compare like for like. Don't try to use a grey-import Samsung in your figures.