The existing ones were quite expensive, especially when I started out. A friend had the idea to get a cheap/non-functioning lawnmower second hand, and tear out the circuit board. We're in the process of coding up a new ROS2 based stack that will roam the lawn on GPS with RTK in the charging station. My friend does most of the electronics stuff, and I focus on the software.

I'm at the point where I will start testing a simple bounding box soon and just have it drive around until it "hits the edge" and then randomly pick a new direction.

It's fun so see the software I build "in real life" instead of as a web-site, as is the case for my my daily job.