Can't believe i hadn't seen this posted yet.
Went to Big W today and grabbed a few sleeping bags (for $2.50 half price) for the bub and saw huggies at a sale price of $24. You will hardly ever see it cheaper (well i haven't yet).
As a rule of thumb, two boxes for every month if you want to stock up. But they are good at returns (full refund, no restocking fee) and swapping for different sizing if your bub grows out of your size.
Is the link incorrect? Blog post is from 2011?