That is not exactly correct. You don't need expensive sensors to synchronize maps with reality. The coarsest estimate of where walls are can be used to infer a position in a map; it will just not be very precise. But errors of cms should be ok.

And Roomba only moves in a plane as a result of its (slow) actuators, whereas an iPhone X tries to do hyper-accurate 6dof real time position, from a very quick external source (your hand), for augmented reality. Thus the sensor and computation cost premium. But that does not apply to the Roomba.

Why does a smartphone have to keep track of a room's boundaries?

So the swat team can find you ?

I think it’s $1,000 more because it’s a powerful, tiny, general-purpose computer and less because it can keep track of a room’s boundaries. That is to say, the Roomba doesn’t need to do everything a smartphone does and thus wouldn’t need $1,000 or smartphone technology.

Xiaomi robot vacuum is a few times cheaper than an average roomba and it had them for years.

well that's not a fair comparison. the iPhone is built to be capable of doing a lot more than a Roomba, thus the cost. Also, much of the specialized expensive equipment is required for the iPhone because of its form factor. For the Roomba, the only new pieces of data one would need is a feed of the current orientation relative to its starting point and rate of travel to mapping out a room's boundaries. The logic is more complex but not to a point where expensive hardware is required.

These things are not exactly cheap either though, even the ones without any mapping.