The worst cost of documentation is that it often generates extra work for you in terms of complaints and updates. Not always reasonable ones, either. More than a few personal projects have been taken offline because it's too demoralizing to give things away and get negativity back.
Considering that some blog post about something can remain relevant for years—as opposed to some off-hand tip given verbally—, it is definitely difficult to just share knowledge in a way that cost you next to nothing. You could give some great advice that works now. But doesn’t in two years. And then it might be counter-productive.
