KJB
Those economics wither when they start taking orders from others rather than following their own muse, so that leaves fair-market compensation.
The economics are different, but not withered.
(For me anyway) I take into account my own interest in the project or feature and usually offer a hefty mark-down for code that’s going back into the OSS pool, and shave my hours on ‘interesting’ projects.
As an existing contributor/maintainer (not a gun-for-hire) I’d probably be spending that spare time doing something pretty similar anyway, so the money is just a (very) nice way of directing my attention to real tasks.
There is a large zone where muse and lucre overlap. I like living there.