[bigstrat2003]

Which it is. Unless you would actually be getting paid for that time (which you wouldn't in most cases), the opportunity cost is $0.

[beng-nl]

Well said. I’m often nonplussed at these calculations of some fairly high hourly rate that we seemingly all should be able to command at will at anytime in unlimited supply. Well, I can’t.

[wdb]

You get paid in happiness of having and doing a hobby that brings peace and joy :))

[dragonwriter]

If you would otherwise be doing anything with positive expected utility in that time, the opportunity cost is >$0.

The fact that the analysis can be carried out in monetary units (because we don't have a good direct measure of utility) doesn't mean that receiving money itself is the only source of utility that needs to be considered.