I've got my bike in a workshop for its 6k maintenance and just got told that its front disk rotor is warped by a few millimeters. They said it should be replaced pretty much straight away and that I shouldn't ride until the problem is fixed. They said it can cost from $100 on the low end to $280 on the high end for a new one. Just wanted to ask and make sure this makes sense and that I'm not getting ripped off price-wise. For what it's worth, I have noticed recently that when I apply the front brake, it "pulsates" [like ABS on a car] whilst coming to a stop. How would the front disk rotor have become warped in the first place? Is it a serious mechanical fault? Something routine? Complete bike nub so I've really got no idea.