You can definitely buy a tux for far less than $100. Some people are hesitant to buy clothes on ebay, but it is not hard - particularly when it comes to a tux or other suit where you know your size.
So what if the tux is used. When you rent a tux you are wearing a used item.
I got one on ebay and it looks great. It was dirt cheap too – including shipping it was under $30. With formal shirt, black tie, and shoes I am still under a hundred. If the tux came and I didn't like it, I would have just thrown it out.
You can buy ex-rentals on ebay, but I was patient and found a (semi)brand name (Saks). I really doubt I would look this good if I rented.