Pingdom - use it on quite a number of projects and they keep adding more data checks around the world making it even better to triangulate back to where and why people are having problems.
An almost trivial one-liner with curl can do this kind of service.
That is the figure you should be looking at because even if you assume only a single day of work you can already buy many months of pingdom service for the same money.
Moreover even the smallest monitoring utility takes quite a bit more effort than a single day once you take SMS alerts with backoff (you don't want to spam, right?), scheduled downtimes and overall babysitting into account. Been there, done that.
So, look at the big picture and those $10 bucks suddenly become a no-brainer.