Glassdoor says that a janitorial salary has a national average of $21,060; compared with $31,200 in San Francisco & San Jose CA; and $24,000 in Wichita KS.

Sounds like Janitors are being paid more in SV.

Since the average starting salary of a coal miner is $60-70k in the rural geographies where coal is mined, I'd say that yes, a coal mine in San Francisco would likely pay $100k.

Probably closer to $75K? If coal miner had a choice (most don't) of whether to work in Ohio for $50k or SF for $75k, I'm assuming they'd pick Ohio.

Thats why someone wouldn't start a coal mine near SF.

People are only going to be paid up to the point where a company still makes money from them. This has a high ceiling for software devs, a low ceiling for janitors.

If being a dev is the new coal mining.

Yes.

It's typically partly removed from your salary: you join a corporate health-insurance plan, whose premiums are partly but not fully subsidized by the company, with the remaining $x/month after subsidies coming from your paycheck. Some googling suggests that the average employee's share of the premium is $1100/yr for single people, and $5300/yr for those with dependent families. The actual numbers vary (quite a bit) by location and company.

It's a little of both. If your employer doesn't cover 100% of the premium then your share is deducted from your pre-tax salary. Healthcare is often one of the factors up for negotiation in salary discussions. It's not a 1-to-1 ratio, but generally if you opt out of an employer's coverage you can try to use that to ask for a higher salary. I've found it's been far more successful the older I get! The reason is employers want young healthy people in their insurance census (young people are cheap to insure and lower the cost for everyone), not middle-aged people with kids.