One example of what you may be thinking of is a radar altimeter. These send a beam, fairly wide, downwards and measure the time delay.
A KRA10 is about £6,000 installed. Still too expensive for most spamcans. OK, the manufacturing cost is about £300 (same as a Garmin 530 for that matter) but that's the way things go...
Detecting a target without a reasonable idea of azimuth is not much use. Which is why the cheap £500 traffic warning boxes are useless, IMHO.