Most of the conversation here is focused on cheap drones. Are there cheap Inertial Navigation Systems (INS)? As I understand, it only appears inside of multi-million dollar cruise/ballistic missiles, fighter jets, and long-range bombers. Please correct me if I am wrong. Also, it might be that there are cheap INS systems that are good enough (e.g., "close only counts in horseshoes and hand grenades"), plus a bit of AI-enabled vision on the drone camera.

Yes, you can buy an IMU as a single IC for less than $5[1]. Otherwise your smartphone probably already have all the required sensors.

Of course, those have significantly less performance than the one you put in an airliner or ballistic missile.

As you mention yourself, its a question of good enough. You need to be a lot more accuracy to hit a city after a twenty minute sub-orbital coast, than to find the nearby trench. And yes, computer vision is used to correct for drift.

[1]: IMU's on DigiKey: https://www.digikey.com/en/products/filter/motion-sensors/im...