How many beneficiaries?
What ages?
Health status?
Pre-existing conditions?
What type of coverage? HMO? PPO?
What area of the country?
Don't forget dental insurance!
I wouldn't even want to guess either. There are just too many variables. But medical insurance isn’t cheap. For an elderly couple, maybe $500/ month or much more. In 99.9% of cases, I would forget about your employer covering them.
When you sponsor an immigrant, the government wants a legally binding contract assuring that you will cover most all expenses for medical care and day-to-day support so that the immigrant does not become a burden on the taxpayers, which seems fair to me.
Now if they were to sneak in from Mexico for example, the bleeding heart liberals would be anxious to offer them every benefit of our society (at the taxpayers' expense of course).

What relationship and ages are these in-laws that you are speaking about? Keep in mind that you can only petition a spouse and her unmarried children. Her parents would take another 5 years or so and siblings could take up to 30 years to get them here depending on the country. Your immigrant wife would have to first earn citizenship through naturalization (4-5 years?) before she could sponsor any of her other family members.
It's best to concentrate on getting the wife here and making the marriage work before you even consider any other in-laws for immigration. I wouldn't even mention the possibility to them at this stage of the game.
Ray