It is hard to factor in what the true costs are since a) my employer is very anti-employee, b) the monthly medical premium is low but coverage is non-existent until after the deductible is 100% met, and c) moving up to a more premium plan actually has a negative ROI. That said, I would have to argue the percentage for everything is about 7% and this is for a company with +250,000 employees.
I am actually getting on my wife's medical this year. I wish there was a way to force my employer to give to me, or to my wife's employer, a portion of their advertised contribution. Instead, they will save $500+/month by my departure. This is why they make the so-called medical benefit a benefit to them, not me.