The cheapest factory-unlocked all-carrier certified phone I'm aware of, $100. (It has bands 66/71)

https://www.amazon.com/TCL-Unlocked-Smartphone-Display-3000mAh/dp/B09B79YT3S

(Similar in size & feel to a Moto E6, but more RAM, and with band 66 that the Moto E6 is missing.)

Used, a factory unlocked Moto E6, with a clean one likely to run $50+.

If you don't need to replace it right away, any of several Tracfone models that can be unlocked after 60 days. Careful picking will get you an all-carrier phone basically identical to the factory-unlocked version, except for bloatware (remove or disable) and Tracfone splashscreen. I'm using the Tracfone variant of the TCL mentioned above, which set me back $30 + $20 plan to reach the unlock date. At $60-$70, you would have several choices.