I can get 20% off on just about anything brand new from one of my dealers w/o even asking. It's a given that he takes off 20%.
15% off on a used piece of gear is a joke and an insult IMHO. I get 15% off postcard coupons in the mail all the time from another local dealer.
I've also negotiated 40% off new MSRP on certain brands in years past but I think those brands 'cleaned up their act' with their dealer network. While excellent products, those brands never held their value because everyone knew they were worth maybe 40% of MSRP to begin with.
