I use AlertSite.com and I have no idea what they charge. I just pay it.
They have some cool features where they can ping your site from various places all over the world and let you know if it's solid from all over. You can also put in some text that you want to make sure appears on the page, say at the bottom of the page, your copyright notice, then if that isn't there (like if someone hacks your site and removes it) then it will set off an alarm.
I can't remember all of the stuff, it was a long time ago we set it up, but it always lets us know when there is a problem.
Make sure your system can send to multiple addresses too. We send email to lots of email addresses when there is a problem - host, cell phones, pagers, home, work, whatever.
Ok, I just looked at their site. Here are the features:
http://www.alertsite.com/features.shtml
Good luck with your project!