What makes you say radar is extremely expensive? Virtually every car from the last decade has at least one, many have two or more. They’re barely more than a PCB and a radar ASIC.

If you want to compete with LIDAR, you need high resolution 4D (range, velocity, azimuth, and height) RADAR. Those are usually phased arrays with expensive phase sensitive electronics, and behind that a chip that can do a lot of Fourier transforms very quickly.

The cheap RADAR devices you're talking about usually only output range and velocity, sometimes for a handful of rather large azimuth slices. That doesn't compete with LIDAR at all.