I bought one for about $460 at homesecuritystore.com. (p.s. I have no affiliation with them that's just the place I bought it). I wanted to go DIY because I didn't want to get locked into a 3-year contract with an alarm monitoring company. Instead I purhcased a pre-paid service plan from Alarm.com via one of their resellers.
I love the Alarm.com service especially being able to arm and disarm it via their iPhone app. I did a review of Alarm.com Interactive Alarm Service which you can check out for more details the Alarm.com services features.
The only thing that I wish the 2GIG system would do is record footage from my IP Security Cameras. I have several of them and it would be nice if 2GiG could integrate a DVR into their alarm panel. Well, it seems they may be doing just that in their upcoming panel. I've seen a picture of their new panel on their website, but I can't find any details except for rumors I have heard that it would include a DVR for recording security camera footage. Given 2GIG's forward thinking on all things home security I can't wait to see their next Generation alarm system. If you want to test out (play with) a 2GIG panel, they actually have a 2GIG Alarm panel simulator in the iTunes App store that you can load up on your iPad. Check out this youtube to learn more about the 2GIG system.