I took this picture after breaking some props, to illustrate the difference material makes.
The green props were cheap plastic replacements bought off Amazon, and extremely brittle. They immediately broke in half when coming into contact with another object. I went through several of these in one session.
The white prop was one of the replacements that came with the quad. It's slightly bendable, so I suspect it's made from some form of nylon or ABS. It survived multiple collisions - most of the time, the edges simply got chipped. The final collision that caused it to tear near the centre didn't even break it completely, unlike the hard plastic ones.
Were the cheap plastic ones worth it? From a cost/durability standpoint, I'd say the "cheap" ones actually end up
costing you more because they break on every collision. Something a little more flexible survives multiple crashes.
I guess it comes down to how much you expect to crash.
From a performance standpoint, I've heard that more flexible props cause stability problems at higher speeds.
Perhaps a consideration if you're racing or doing pictures/video.