Circles.Life are running a temporary discount that nets you 23GBs of data for just $18.
The 20GB no-contract plan, which is normally $28 a month, can be reduced to $18 a month by using the promo code 10FOR12 when you sign up. The promotion runs until November 30th and the $10/per-month discount lasts for the first twelve months you're with the MVNO.
In addition to the 20GB, Circles plan also includes 3GB of "bill shock protection" which kicks in free of charge if you go over your allowance, so the plan technically has 23GB per month. If you go over that, you can purchase extra data packs.
Here's how Circle's latest deal compares to similar SIM-only plans from their competition: