Another problem with doing your own sysadmin automation
In addition to the stuff in AutomationCosts, there's another problem (or cost) with custom sysadmin tools: they're almost always at least not-good software, and sometimes they're outright bad. They pretty much can't help but be because of how they're created.
The typical custom tool is written by at most a small group of people (who are often not actively practicing programmers), is not looked over by very many people, and on top of that is often written in a bad language for programming. I'm not saying that you have to be an active programmer to write good programs, but I think that it helps; the odds are stacked against a small team that doesn't do much programming producing something good. Similarly, having lots of people poke at your work helps improve its quality, especially if they are outside people.
(Yes, there are sysadmins who are good, active programmers, but generally the people who really like programming become programmers, not sysadmins.)
In short, your custom tools just don't get the degree of attention and involvement that vendor software or open source software gets, which stacks the odds against them being as good as such software.
The usual rejoinder to this depressing view is that you'll generalize your custom tools and then share them for the world to adopt and improve, which will get you all the benefits of open source software in general. Unfortunately this does not seem to work in practice; the LISA conference proceedings are littered with tools that have gone basically nowhere since being announced. You might be an exception, but history suggests that the odds are still stacked against you.