Back in the day I was very penny-wise and pound-foolish -- it was practically my tagline.  In some ways I still am.  But I've learned about reimplementing stuff outside my core competencies.<p>It doesn't matter that $100 or whatever sounds like a lot of money to me, or that $100 can buy a lot of slices or software or pizza for that matter.  What does matter is that setting up cacti/nagios/etc takes time.  That is time where I could be doing things that actually make money, like fun SEO work or intellectual-stimulating-as-watching-paint-dry-but-sure-as-heck-profitable button A/B testing.<p>A single A/B test which offered a 3% lift <i>anywhere</i> pays for $100 a month, and for companies or individuals operating above my scale, you have even less of a bar to waltz over.  I also typically don't have to maintain those (gif files break remarkably seldom in my experience compared to software), where getting more software running just increases the number of things that can need tweaking or go wrong next month.<p>Now, you're almost certainly a better sysadmin than I am, so maybe knocking together some cacti/nagios scripts is really, really fast for you.  But I'm betting the best use of your time is probably more effective than doing this.