My ex-spouse called my place of employment and inquired about how much I pay for single and wanted to know the cost of family is my ex-spouse allowed to do that?

Why wouldn't she be allowed to do that? She could probably get a court to order you to provide the info since it probably pertains to support issues. The better question to ask is whether your employer is allowed to give out the information.

There is no law preventing her from calling and asking for the information. I'm not sure if there is even a law preventing them from giving it out, but few employers would just give out such information.

Unless they are a dependent on your insurance, I am sure your place of work will not give it out. There is no law that says he can not ask.

If this is dealing with support payments, or dealing with child support payments for medical care, then he may have a legal right to know, and can find out, though a court order also