I'm with ADT at $26/month. We bought a few extra sensors and such which increased our upfront cost, but to me the biggest issue is monitoring cost. Keep that as low as possible.
I had a guy come after the fact and install a card in my system that cost me $100 (think third party, or DIY). It's from a company called Connect2Go, which basically allows me to get alerts and set the alarm from my phone. Added no additional monthly cost to my system, and ADT wouldn't even know its there.
http://www.connect2go.com/