We accept Amex where I work, and it doesnt matter if your card is Green, Gold, Platinum or Black, it doesnt cost us less or more.
The only difference is with Platinum & Black cardholders, as we try to provide a higher level of service to them (I dont mean this to sound discriminatory, as every customer is treated with respect and due courtesy, just with the high tier cards, and given that most of our transactions are quite sizeable, we do try and go above and beyond as normally these particular cardholders represent the buying division of large national or international companies that we deal with).