All good comments above and 30 Euro for part and install is reasonable; however, I agree with a couple of posts that the better quality rotor "generally" the fewer problems down the road, better longevity and better performance. The, "Buy once, cry once" comes in to play here...within reason. As far as availability on-line, do some homework and read some reviews, lot's of opinions out there...

Me personally? I have Shimano "Ice-Tech" rotors on my one bike with disc brakes. I love them but they aren't cheap depending on which ones you look at.