In my experience with running nagios it's never as easy as set and forget to keep your monitoring system effective. Even more so when you're growing or scaling and systems are changing more regularly. To me that $11,388 per year is cheap compared to the costs of having a salaried employee spending more time on a slower to implement solution.

If you don't have a full time sys admin it's also one of those things that is very easy to put off or delay or just not keep on top of.

As for the data loss I can't think of a time I've ever wanted to see resource statistics for over a year ago and down times and outages are kept in a problem tracking system.