While the other answers try to quantify the value of health care the question you ask is about employee vs contractor. The delta between those regarding benefits goes way beyond health care. In fact because almost every full time employee must have health care offered by their employer the option of "you can have X with healthcare, or Y with no healthcare" is no longer an option.
I have seen situations in the last few years where employees who had no need for healthcare coverage (retired military) were offered additional vacation days to compensate for their lower cost to the employer.
For employee vs contractor what is different isn't just healthcare. It also includes holidays, vacation days, sick days, employer portion of social security, education benefits, and 401k. Insurance benefits include not just healthcare but also dental, vision, short term and long term disability, and life insurance.
The rule of thumb to cover all these benefits that are lost when you are a contractor is an amount equal to your income. Of course some of these benefits depend on single vs married and kids or not. But unless the rate they are paying the contractors is approaching twice the rate they are paying employees the contractor will be hard pressed to cover the missing benefits.
