I tried a cheaper rotor on my Avids. Though advertised as the same diameter, it was smaller to such an extent that there was less pad contact. That would have caused a bad wear pattern not to mention less braking. It was also poorly finished. I threw it away.
As already mentioned, an extra disc is a good idea. I keep a lot of spare parts as I have to maintain four bikes and my wife and I go off on bike trips.
Al