A decent robot mower will set you back a couple thousand dollars, considerably more than the robot vacuum that's patrolling your living room for dust. But if you've got access to a 3D printer and a bit of electronics know-how, you can print and build a robotic mower for a fraction of the cost.
You'll still have to supply all the parts needed to build the Ardumower, including PLA plastic, three electric motors to power the wheels and cutting blade, a couple of Arduinos as well as about $US12 ($16) for the building plans and engineering schematics. But in the long run you'll still be saving yourself thousands of dollars, plus the ten bucks you've been giving the neighbour's kid to cut your lawn.
So that it remains cheap, instead of using complex software algorithms, sensors and cameras to ensure the mower only hacks away at your grass, you'll also need to install an underground wire surrounding your lawns that serves as an invisible boundary for the Ardumower. When it encounters it, the mower will automatically change direction and continue on with its task.
Just be aware that a robot criss-crossing your lawn rarely results in a particularly neat finish. There's a reason golf courses and baseball stadiums still rely on humans for the task. But who cares what the neighbours think if it's one less item on your weekly to-do list?