Well, that's interesting. My dad works for a union and only pays something like $20 per month in dues.
Of course, you're only working 9.5 hours per week and dues are a fixed rate, so if you worked more, you'd get a higher percentage of your pay (which of course, is realy hard to do for a full time student, I know, heh...).
I'd bring it to your management's attention. If they can't do anything about it, well it's up to you if 23.83/9.5 = $2.50 per hour is worth it. (Heh...)