Cost varies.
I bought the top model in 2005 and paid $1600, at the time. Now that same mower is going for around $1200 I quess, new.
You can also buy used for less.
also, you don't have to have the top of the line. The other models, which don't come with the AUTOmatic charging station, mow the same, but cost less. Just set it in the yard and let it do its thing. When they are done or battery runs low, it returns to where you set him down.
I've seen these on Ebay and the like, going for a couple hundred.
They are really cool. So quiet, no gas, no air pollution, no uneven cut, no clippings to deal with. .
Get rid of that outdated fossil fuel burning engine. Its over a century old, and we are all paying the price in compromised health and quality of life issues.
And these mowers are so safe. They have a bumper all the way around the mower that has a safety switch in it that instantly turns off the blades. I doubt any push or riding mower has that. I laugh when I see people sitting on these rediculous, noisey, riding mowers. Some people are smart enough to wear hearing protection, but then there is the gas fumes and heat that comes off the engine. And if you are sitting behind that, yuck. When I had a lawnboy mower, I always counted on taking a shower right after to get the fumes and dust off me.
That is no problem with a robotic mower.
Another option is to have a herd of sheep. That is the ultimate in natural grass length control.
http://cgi.ebay.com/Friendly-Roboti...tem&pt=LH_DefaultDomain_0&hash=item45f3a24136