> I've personally offered to work at startups in the evening for about 50% of my hourly-equivalent rate for 20 hours per week (so 1/4 of a salary for 1/2 of an employee), and they've not been interested.

I don't know what your role is, but I can understand why they aren't interested. If you're only willing to work at off hours, remotely, then it's highly likely they could get someone as good or better, more cheaply, by getting remote offshore talent (I worked with some great offshore devs in Brazil and Eastern Europe, for example).

In other words, if you've eliminated any advantage you would bring to the company as a local or US-based developer (assuming you're in the US), then your salary comps are now global developer salaries, not California salaries or wherever else you may live.