There are already very good sub-$100 lidars, especially for 2D since they were made en masse for vacuum cleaners. E.g. the LD19 or STL-19P as they're calling it now for some reason. You need to pair them with serious compute to run AMCL with them, plus actuation (though ST3215s are cheap and easy to integrate now too) and control for that actuation which also wants its own compute, plus a battery, etc. the costs quickly add up. Robotics is expensive regardless of how cheap components get.