I'm aware of NextAlarm and DollarAlarm but I don't consider them the equivalent of traditional alarms.
The Vonage Forum has a lot of complaints about NextAlarm incompatibility with their Broadband setup, and NextAlarm only uses Broadband as the communication path.
For me it's worth having redundancy for the safety of my family. I have smoke alarms and perimeter sensors that I want to have constant contact with a central monitoring unit all the time. NextAlarm is cheaper at $8 or $9 per month but the lack of redundancy and the noted compatibility problems with broadband is not attractive to me.
This is where I believe money spent wisely can save a life.