The MLB.tv Premium subscription is $129.99 and it includes the At Bat app.
You also get home and away broadcasts. If you have a favorite announcer this is perfect. For me, I'm a huge Vin Scully fan. (He's returning for his 66th season in the Dodgers booth. God bless him.) You learn a lot from your TV announcers. AwfulAnnouncing.com produced The MLB Local TV Announcer Rankings last year and if you're interested (and I know you are) you can see it here.
Another huge benefit to having MLB.tv Premium subscription is that it supports over 400 devices from Xbox 360 to Chromecast to Roku. It's rather ideal if you want to watch out of market MLB games on your favorite devices.
For more information you can visit MLB.tv. I hope you enjoy the upcoming baseball season and let's play ball!