Agreed on RTK cost. Two RTK receivers cost about the same as a commercial robot lawn mower.

I have half-finished contraption on my garage desk, consisting of hoverboard electronics and motors (cheapest battery, motor, power electronics combo), PX4 autopilot and a spinning disc with Husqvarna razorblades as cutter. Time will tell if I muster enough energy to deal with the rest of the plan this summer- Raspberry Pi running ROS and Kinect for SLAM.

Right now though, it's just a scary RC vehicle capable of doing wheelies (hoverboard motors have a lot of torque).

what's affordable? You can buy corrections and a $300 GNSS receiver or buy two $300 GNSS receivers. That's not too bad for something like a robot mower.