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.
I think the difference is that these are intended for automotive use and have a much longer range than the ones in your Roomba.
True, you have to go up to $120 for the 25m version, or $450 for Unitree's L2 which gets 30m in 3D. That's about as much you could possibly ever need unless you're making high speed vehicles that need more reaction time. In which case you probably shouldn't be relying on the cheapest thing on the market :)