it really depends. like i said earlier on, if you're just looking to get a degree, the textbook MIGHT not be a must. if you're looking for distinctions, then probably by sem 4 u'll realise how useful the textbook will be.
my friends used to be doing without the textbooks while i used it, and this sem i did better than most of them(except for that RJ friend; i didnt get to ask him). their conclusion is that they're all going to get the txtbks as well, not because of the difference in our results, but because sem 5 was a warning for us that things arent going to be easy from now, and having come thus far, we all want our degree with distinction.
if you've no background in biz finance or risk mgmt, or arent strong in it, please get the textbooks. i've seen ppl who failed these 2 subjects. (though it might be too early to tell u this, as you're a yr 1 student, but just fyi.)
edit: FWIW, you might try attending the first week's lecture by the rmit lecturer, if by then you can understand and are able to answer like 95% of the tutorial questions up to where they stopped, you should be fine without the textbook. (considering that the first week is usually the simplest and the fundamental topics, 95% isn't too much to ask for.)