Online deal only.
Following the Optus sim $10 deal for the $30 Starter pack, I had a search on the Optus website and found a deal I've bought before.
For music lovers, Optus's $30 Prepaid recharge (and above), for 28 days, includes music streaming (e.g Spotify). Can't find any other prepaid that is cheaper that includes free Spotify data streaming.
For those who don't do much calling but need 1GB of data, OVO Prepaid $9.95 per 30 days is the cheapest option I've found on a "month by month" payment with 1GB of data:
https://www.ovo.com.au/sign-up?pc=OVO_10_DV
That's weird, the banner says half price but when I click on it, it wants to charge me $40.
edit: When I click the link above it works. Even weirder.
edit2: bought one, on the receipt page is has $20 crossed out and $0.00 next to it. Something funny going on at Optus!