This problem has been addressed over the last 20 years by more products and services than I can remember, ranging from csh scripts that would ping a site and send an email if it was down to HP OpenView.

There are more than a few and sub $100 tools for Windows that'll do this and a lot more, so charging $15 for a service isn't going to fly.

Further, to properly monitor uptime requires a lot more than just monitoring the host. You need to monitor the various services, and the health of those services. Is the web server returning a valid page or a 404?

Then there's the need to monitor hosts and services that are not publicly routable, getting into authentication or the need to run the service inside the firewall rather than via a public site.