Robotic lawn mowers are nothing new. They have gotten more sophisticated over the years but still cost a lot of money. You could always build your own though. The Ardumower is a 3D printed lawn mower that you can build on your own. It has two 12V geared motors and a rechargeable battery. You are going to need an Arduino to get started.
The robot can detect boundaries and mow your lawn continuously. It can handle up to 500m2 of lawn. The above video shows this robot in action.