Truly a la carte data is expensive. Best I've come across are monthly cost SIMs that allow you to upgrade/downgrade your plan as needed. US Mobile seems to be the best for this as you can drop to a $2.50 plan with no data if you just want to keep the SIM active, though they're T-Mobile or Verizon only.
For AT&T you could try PureTalk: they sell trial SIMs to play around with: https://www.amazon.com/dp/B07NFCKQ17/