When I access my carrier plan options right now (in settings on my LTE iPad), there is a Tmobile prepaid $10+tax per month option for 2GB per month. I imagine with taxes that would really be more like $13-16. I've tried to understand the comparative options on the Verizon and AT&T websites, and that seems easily the best for budget usage. Especially since it looks completely independent of your regular carrier bill.
The plan I actually pay for as an AT&T customer is "$10/mo" (~$14/mo with tax) to add my iPad to my shared family data plan. So about the same as that standalone tmo plan but instead of 2GB/mo it just pulls from the same shared 30GB/mo as my family's iPhones.
But man that $200 cellular fee is steep for useless 5G. I'm in a suburb of a major city (Austin, TX), and my LTE devices often struggle to reach 3Mb/s download speeds. Apple wants me to pay for 5G when I'm still waiting on LTE speeds to roll out?