Radar altimeters are widely deployed for drones and available for less than $100, eg https://www.aliexpress.com/item/1005012484730308.html

Ardupilot boards are $20 and support lots of different altimeters technologies: https://ardupilot.org/copter/docs/common-rangefinder-landing...

Or something newer, like this one: https://www.garmin.com/en-US/p/166298/#specs