In addition to not valuing yourself too low, there is the argument that, the more you work for other clients (whether that is to your employer or your clients as a freelancer, etc), the scarcer your time outside of working for those clients actually are! When supply decreases, the price increases! Another great reason that selling yourself (or buying yourself) your own time is another great investment, even if the short term cash isn’t immediate (as in the case of your example of writing blog posts).
I also just finished up reading Snowball and it was a fantastic read. Such a life worth emulating. I’m starting to write about some of my lessons from his book.
Nik, have you read Seeking Wisdom by Peter Bevelin? I’m gonna be starting that one soon.