Self-monitoring (via email/SMS) is free (aside from the upfront cost of EnvisLink and alarm system hardware). Central monitoring is available as well.
Disclaimer: I have an EnvisaLink 3 hooked up to a Vista 20p (ADT Safewatch Pro) system monitored by a ULC alarm facility. ADT kept hiking up rates and wanted $30 per month after the initial system was "sold free". Ashamed to say that I paid a monthly fee for almost 5 years before switching to the EnvisaLink setup. You need to know (to a certain degree) how to program your alarm panel.