Where are you shopping that you can find a 20" touch screen for under a dollar?
Sure, touchscreen is expensive, but if you just have notification on your phone that the door is not closed properly that's already considered "smart". Basically anything connecting to your home network is advertised as such.
> app and infrastructure are not rocket science
Thanks to Tuya, the software and infra side for making a garbage-quality IoT device is effectively free now. You may be surprised just how much of those you have around you. Any random device class that suddenly gained optional "control through app/voice assistants" feature is likely Tuya in disguise - magic of white-label IoT.
Those will soon also not require you to connect them to your wifi, NB-IoT both chips and plans cost for businesses are already very good.