Employers cover a majority of the cost of insurance. In Boston, I pay maybe $70 every 2 weeks for health insurance, but on the private market I'd be paying 5-10x as much for worse coverage.
A law called ERISA makes this unlikely. Companies must offer benefits like health insurance and retirement savings plans on the same terms to similarly situated employees.
I've never heard of that, it would cost you more than it would cost the employer since you first have to pay taxes on your income, and employers can often get better rates.