I have never seen them on sale, but you can get 20% off at Bloomie's discount days where the whole store is 20% off or Nordstrom also does 10% off the whole store sometimes. Another option would be to open a store charge, sometimes you get a discount.
I have never been able to get a discount on Chanel sunnies- I never see any on sale in the Chanel boutique, and at my local small sunglass boutiques that do carry them- they are not allowed to give a discount on them per Chanel's request. Believe me, I have asked more than once lol! It is the only brand the sunglass boutiques/eyeglass shops cannot give a discount on...it has always been like that.