It's not hard to check union dues avg a few percent of income per year. Over a 5 year job this is an order of magnitude more paid out of your salary than the once paid alternative listed here.
The collective weight of the union as a counterweight to employer power and overreach is also wirth something. In a dispute with your employer, HR is never on your side, but the union is.
People need to stop going to HR with their problems, in general. A nice letter from whoever did your contract review works a lot better if you aren't at a union shop, and also isn't very expensive.
The 1% is from the comment I responded to (and is actually higher than my past experience - employment contracts are really easy to read). The 3% is from an average of union dues.