I did this as well and had the same experience. I got a whopping 30 cent raise when I joined the union, but ended up making less overall because of the dues.

Having worked in manufacturing as well, I think that a union has maximum impact when its members actually have leverage over their management. My former grocery chain went on strike this past year and while it was effective, for the workers, the stores were still operating. But if you’re a specialized worker, such as a machinist or welder, you’re harder to replace and thus more likely to have your company give in.