Well if they knew right off the bat you had a GC then they're probably just playing hard ball. Margins on tvs are definitely high enough for them to handle a $200 discount.
I know they usually only haggle with people if the person wants to get an extended warranty or a high-margin item like a Monster cable product. That way the customer THINKS hes getting a deal, but really FS is probably making even more money compared to if the person just bought the products by themselves.
My suggestion to you would be to go in and throw in a bunch of $200 HDMI cables in your cart, try the same sort of discount, and then return the next day and return the HDMI cables. They will be more willing to deal with you since Monster HDMI cables have astronomical margins.
FYI ive seen the Brick advertise a $100 Monster HDMI cable that came with a FREE Samsung DVD player. That gives you an idea of the margins. Also, when PS3s first came out, and they had to put peoples names on a call-back list, they would only do so if you agreed to buy the warranty.
Sony KDS-60A2000, Sony Playstation 1/2/3 (60gig) and PSP, Sony STR-DG910, Pioneer/Sony 7.1, 39 PS3 Games, 120 BR movies