>What makes lidar for use in self-driving that expensive? I have a robot-vacuum with a lidar that wasn't all that expensive.

Waymo says it cost $75,000 for the lidar in one car a decade ago, and in 2019 they said they got the cost down to $7,500. So at the time Tesla decided to forgo lidar the costs were certainly very high. I'm guessing the range in your vacuum is multiple orders of magnitude less than a car.

Waymo must have been betting costs would drastically fall over time. No idea if it's a lot less than 7500 now.

Robot vacuums typically don't use Lidar. They user a laser pointer and an offset camera. Distance is estimated by the pixel offset between the laser dot, and the center of the image.

Object very far away: Laser dot in center of image. Object very close: Laser dot at maximal offset from center of image.