I can buy a Complete Screaming Eagle Compensator that comes with a New Rotor for $249.
And that is MSRP. It can be gotten for $200 flat.
Unless your year/model is "one of a kind" and the part fits only 1 year, I can not see just the Rotor (part only) costing $400.
The word "kit" may include the Stator which he will keep because yours is still Good..
Something is just not right. Go to a parts manual and look up the part number of your rotor NOT as a kit.
The other part I don't understand is the "slipping" of the rotor. If it was slipping, the noise would be terrible. I realize you only wanted a source for a lower cost part so you did not give details on why you were replacing it/charging issue/regulator etc but the whole thing just makes no sense to me.
Now our friends in Canada may have to pay that price. Is that where you are from.
I saw for myself that the rotor was not turning the same as the crank. He showed me an old one and said that it was probably damaged on the spline part of the bore. The issue is charging and there was a feeling of vibration from the left side which would go along with the rotor issue. And yes up here in Canada we do seem to get hosed on parts, that is why many us turn cross border rides into parts pick up days as well