A SIM card is normally $5 but right now you can get one for free: https://www.att.com/cellphones/prepaidphones/sim-card-kit-phone-prepaid.html#freesimcard. Note the 1GB plan is $35. The $30 price you see on the website includes a $5 discount if you have it set up on autopay with a bank account or debit card. If you do that, make sure you disable autopay before you leave the US. You won’t be able to turn it off after you leave and you don’t want to continue paying for service after you leave.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.