Whether you must join the union depends on the situation between that union and that company. Your cost would be whatever that union makes you pay. You’ll need to get the specifics directly from someone.
(Now, as you are considering this unionized job…
Some would say that unions destroyed the American auto industry, which is food for thought to say the least. There’s no logical reason a car should cost $30,000. One reason they do is because unions force the companies to pay workers $75/hr for work that’s only genuinely worth $20/hr. Anyway, that’s a whole issue I don’t want to get into.
My preference has always been to work for smaller companies, because I can work hard, get noticed, get promoted, and get raises. In a union it’s all politics. It doesn’t matter how hard you work, as “seniority” determines everything. The longer you stick around, the higher you climb in rank, the more you get paid, and the less you have to care. That’s my experience in real situations. I have friends who laugh about how little they do and how much they get paid. Sure, it’s a joke that you’re getting paid twice what you should be. What they don’t understand is it means everyone has to pay twice what they should be for the products they make. And that’s just stupid. I once had friends working for the city (unionized of course) who would cut grass for an hour in the morning, then sit around for the other 7 hours doing nothing. What goes around comes around; people must be paid for the value they create, or the larger economy will never be in balance.
Sorry for the mini-rant, all I’m saying is if you want lots of money at any cost, and don’t care about the ramifications of it, then get into a union as soon as you can, keep your head down and don’t do anything to get fired. For a chance to be noticed, innovate, grow, and be rewarded for what you are really capable of, you may want to consider a situation with more freedom.
Then again I work in Canada, and get full benefits (dental, medical, optical, etc) with any job, regardless of the size of the company. I can’t imagine extorting my own employer to pay me more than I’m worth, or to simply treat me properly. This gives me the freedom to find work that’s truly enjoyable without worrying about the basics. It’s awesome, that’s for sure.)