In general, US employers contribute to their employees' health insurance, unemployment, pension, etc. Equity has health insurance and pension funds for their members, so employers contribute a specific amount to those - one is a percentage, one is a dollar amount. I don't remember which is which, but you can get those numbers from the Equity Broadway Production Contract, which is available on the Equity website.
Most unions don't put their agreements online for everyone to view, but Equity does, so take advantage of that. It would at least give you a guide of what the employer has to pay for each Equity member, although that does not cover the federal/state/city tax contribution or the unemployment insurance contribution, which are percentages of income and set by the government(s), not the union. It also does not cover the insurance policies that the employer has to have in place, although that does not directly benefit the individual employees (hopefully), and it generally covers everyone working in the theater, including orchestra, stagehands, etc. We can't really give you numbers for those insurance policies, because they are determined by hundreds of factors and set by the various insurance companies.
Slightly unrelated, but I highly doubt that Nathan Lane and Matthew Broderick were pulling down $80,000 a week each, even when they returned. Maybe $40k each. Usually, a big star in a show negotiates for a chunk of the box office income on top of their salary. That way they do well if the show does well, but if the show doesn't do well, the producer isn't run into the ground. The most I've heard of an actor getting was $35,000 per week plus a percentage of the box office, and that was an A-list Oscar winning actor in the starring role of a high profile revival.
Back on topic, I appreciate your question. Most people don't understand how much an employee really costs to an employer, in any industry. It's more than just the salary, and people don't realize that.
Nothing matters but knowing nothing matters. ~ Wicked
Everything in life is only for now. ~ Avenue Q
There is no future, there is no past. I live this moment as my last. ~ Rent
"He's a tramp, but I love him."