Make a list of somewhat rare words like defenestrate (I love that word <3), inculcate, microcosm, hegenomy, sacrosanct (that one's actually from VCAA)
Unless every single word on your list is in the dictionary, don't buy it, because you probably won't find it useful. It's a logical approach; you will only use it for words you aren't familiar with, so a dictionary with only more common words is less likely to be helpful when you most need it. Most 'concise' dictionaries are fairly thick and cover all but the rarest vocabulary. Avoid a learner's dictionary, they're not for native speakers hence don't contain complex words.
If you don't want to spend a lot of money on a dictionary you'll hardly use, find a non-year 12 friend who's got a good one that you could borrow for the exam - don't go into the exam without a dictionary, it could be a lifesaver. My cousin finished year 12 last year, and the only dictionaries I have are children's dictionaries from when I was little and a HUGE oxford english dictionary that my brother won for coming first in a national English comp in year 5, which is just way too big to lug around or take into an exam. So I borrowed my cousin's exam, saving myself ~$60 in the process.