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.
$490 AUD for the kit, and I still need to buy the original mower to modify...
The original mower will cost $1165 AUD
So $1655 for the whole solution...
Pretty sure it's targeted primarily at a small group of enthusiasts so with a low production volume, the high price makes perfect sense. If that's too much for you, just don't buy it :P
Seriously, don't be a jerk.