I think it would depend on my personal circumstances. As an alumnus of a private school and my parents forking out thousands more for tuition, I don't think I would've used premium resources. It's not so much the $20 when you're already spending thousands--it's just more the fact that I could probably have gotten the answer elsewhere anyway. So no in the situation I was in. And premium resources didn't come on till after I had finished my HSC so there was no issue.
But if I had little support (whether school, or lack of outside tutoring, whatever), then I would probably have used premium resources. I don't think $20 is too much to ask for a year's worth of resources. And it would also depend what subjects I was doing and which ones I had difficulty with (hence the use of premium resources). e.g. I did Music 2 so I doubt there'd be any resources on that one (sorry if there is, haven't checked).