I use
iSmartAlarm for the past 3 years. It is all self monitored which means you do not pay for a monitoring service. (Here you go Gonzo42-->)So I pay $0/month for monitoring. Really hard to beat that price.
Best value for the money, especially since late 2016 which gives you control over each sensor.
The downside is that they don't have smoke detectors. I use Roost for the fire, smoke, CO detectors. Those alert on my phone like iSmartAlarm does for intrusions.
One thing I found out about Roost. If the alarm doesn't use a 9 volt battery (Roost replaces that battery) you put the Roost inside or near the horn on your alarm and it will work just fine. The Roost has a microphone built into its battery and it listens for the warble of the alarm horn. Pretty clever.
I have 3,500 sqft house single story. I have about $600 invested in the entire system ($350 when I started with the basic system). As I added more sensors to better cover the house I got to where I am now. I will probably add more cameras in the future.
My iSmartAlarm has worked almost flawlessly. It has limitations and if you can live with them it is a great alarm system. It is inexpensive and is literally as easy as their videos claim it is to set up.
Some people have complained about it not having a battery backup. $50 at a Best Buy and you can add an off-the-shelf battery to your system.
This video is pretty funny, but it really is that easy to set up.
Man vs. Woman (iSmartAlarm install)