This actually just happen to me on my 325xi....
All the way back in January I was stopped at a stop light and heard a loud shot, like a high tension wire breaking. I had no idea what it was, but about 2 weeks ago I was rotating my tires and noticed the rear passenger spring was broken. So I have been driving unknowingly with a broken spring for about 5 months. I wouldn't recommend driving on it though, I would have changed mine sooner if I would have known.
Like you, I also started looking for a set of stock springs for my xi. Thinking used would be way cheaper, I ended up paying $95/spring from Bavarian Auto Recycling. Of course the next day Tischer emailed me the same quote as you. I really wish I would have bought two new springs from Tischer. So to answer your second question, yes, I think $135 is reasonable and I would highly recommend just buying a new set from Tischer.
Hope that helps!