I normally only come across truly roached, threadbare cook grates on the smaller used Webers. I guess they sit longer around moisture and have less frequent usage?
Point being, nickel-plated cook grates will always rust due to heat and moisture. Concentrated heat is what the Vortex supplies. That's not to say the center would get destroyed right away, but I'd expect the center to be worn sooner.
As an alternative to buying a GBS, you could be on the lookout for any sales of regular cook grates (or grills) and just consider them sacrificial.