We have ADT and pay about $42/month with taxes. Installation *I think* was about $400 or so. We've never had a problem with them, and I like the system. I'm also able to control it through a smartphone, and there weren't any additional fees for that, which I liked.
We purchased Nest Cameras outright, and setup them on our own. I never did a cost comparison, but from my Google Searches I think ADT charges more than Nest does for Video Storage, and we were able to purchase our cameras during a Black Friday sale, which saved additional money. We also purchased our own smart lock, and DH set it up.
With that said... when you are comparing security companies, I can not urge you enough to do your research. For whatever reason, a lot of these big companies like Brinks, Vivint, ADT are in bed with "Authorized Dealers" which can be completely hit or miss. A lot of them will promise the world - and completely underdeliver. I highly recommend researching any "Authorized Dealer" with the BBB.org, Yelp, Google Reviews, and see what complaints show up. Some are very good companies and will give you a better deal - but when I was shopping around, I felt like I was being a little deceived and the sales tactics were overly pushy.
Make sure you ask about the monitoring contract length as well, when doing the comparison. One local company here wanted to sign us for Seven Years... and we had originally planned to move after Three, so that would have been horrible.
I'd also consider asking neighbors for recommendations.
