your dad's should be accepted - my mum's is and her's is ancient too - it'[s al ife membership,
but why not get your own? it's only $25 or so and its a lifetime discount.
Law textbooks, well, a lot of units you have 2 - one with the cases etc, one with commentary. Some have just one. Some like law foundation have a couple + a reader. Lots have readers with additional information.
So Law textbooks per unit will range from say, $100-$220 ish.
That's for the "text" ones. Often you'll want to get recommended ones too, but don't buy them until you have your first class because often the teacher says they're pretty useless; otehrwise wait a few weeks to see if you'll actually use it.
Business ones - business textbooks are usualy $60-100. If its a custom publication usually $60-80. If its a reader, $28-$40 ish.
Usually they're on teh cheaper side though (business ones at least).
But after all that's said - go to the co-op site and check it out - prices and texts you'll need are there
and then if you don't like paying that much, check out textbookexchange.com.au or lawbooks.com.au - the first is 2nd hand, the 2nd is new but both are lots cheaper.
http://www.coop-bookshop.com.au/bookshop?basket_name=1040794&user_id=1170104392823&_frames_=no&_next_page_=DataSubjSelect