I don't think it costs anything extra beyond your subscription. I just downloaded one movie to test. No mention of price. And once you start to view the movie it expires in 48 hours. Not bad.
I've been downloading movies from Itunes and that can cost anywhere from 99 cents to $7 a pop. Their downloads will stay in your computer for 30 days but once you start the movie to play you have 24 hours, then expires.
And yes, you can download onto your laptop, or any device with a Netflix app.