This is what I've got:

https://www.amazon.com/dp/B0B5GNT8VM?ref_=cm_sw_r_apan_dp_058XC7JV84QCNZDJ6QCQ

It was on sale for $99 at the time but the price has since gone up. Just searching Amazon for "portable monitor" however shows a number of sub-$100 options. Just make sure you check the specs and make sure it supports HDMI and/or USB-C depending on how you plan on using it, and that you have the appropriate cables (the HDMI input on mine was mini-HDMI for example so I needed an adapter).