Like many things in life, it totally depends on size. The size of your union, that is.
The head of the Teamsters probably makes a million bucks a year. (Not a very comradely sum but…it's a union—the rules are different.)
The head of Local 302, Los Altos Plumbers, probably makes just enough from doing that job to pay for the gas in his car to get to the meetings each month. Union dues cover the administration of the union and your salary, as head, is a big part of that.
In fact, it isn't only the members of the union who are required to pay those dues. If you are not in a "Right to Work" state, other employees may have to pay them as well, simply because they benefit from the rights and pay that the union negotiated for all workers as a whole. Depending on where you're doing your…unioning, the monthly dues generally range from between $50-70…you can imagine that if you're getting a sizeable percentage of that amount from, say, 40-50 employees every month, you're going to be doing all right for yourself.
For your average union, the expected annual salary is somewhere in the neighborhood of $200k-$300k. Which ain't bad for a middleman.