129
I've been following the project for a year now and Clemens Elflein has actually managed to get the topic off the ground professionally. Recently, his kit is available, with which you can upgrade a cheap Yardforce robotic mower to GPS support. This eliminates the need to lay boundary wires.
If you are interested in RTK and the practical aspects of the hardware use, check out https://discourse.agopengps.com for a bunch of farmers that use it on an opensource autosteer system.
I also have a group buy running there for the Micro ardusimple units going that's waiting to hit 50 units for a substantial discount if you're in no hurry to get one for your project.