Cheapest Unlimited Data (Speed Limited) Plan

Trying to find a suitable plan for my teenage kids. I'm doing the sim shuffle each month and TBH such a PITA.

They're on Vodafone at the moment with a $9 for $30 starter pack. This is giving 40GB (normally 15GB) at full speed then unlimited data at 1.5mbps.

Which is fine. They only use their phones and videos/tiktok/insta etc is all fine at that speed. But $30x3 is too much. [Edit: Just realised if I do auto-recharge it drops down to $25. Still more than I want to pay but so far found nothing better.]

Are there alternatives I haven't considered?