GMC/Chevy GMS pricing?

I am looking to get a Sierra EV or Silverado EV in the near future. I get GMS pricing through my company and Costco. Is this going to be the best pricing I can get or is there something better that can be had through negotiations/broker ?

Does anyone know how I can find the GMS pricing on these vehicles?

On evs gms is a discount.
This is a discount before incentives and dealer discount

Costco is usually separated as well