For a reported $1 billion with a big ol' B.
All puns preemptively aside, this is a really big deal. Twitch already consumes an incredible amount of traffic, and this acquisition will bring even more video game goodness to the mainstream. With a strong financial foundation, and a backend that can support the ever-growing traffic, this could be exactly what Twitch needs (in before "omg Google = evil empire, G+ comments, lolololol #theworst). Intriguing.
Flashback to 2001, if you told 15 year-old me that when I grew up, people would routinely watch other people play video games on the Internet, and the people playing said games for the people watching them made money from it, and IT WAS COOL? I would say something along the lines about how much I didn't believe you, but that the future seemed like it was pretty much Heaven, but you wouldn't really be able to hear what I said between the muffled sobbing coming from the locker I was stuffed in.
Update: I altered the headline to more closely resemble the situation as it is currently being reported. Shout out to Reddit user AredditorAMA, who is most assuredly not a dick.