My wife and I are big fans of the Discovery, the 04 being the last of the old school Land Rovers available in the US. I suspect that is partly due to an unrealistic view of ourselves as being as likely to be off-road as on: this is definitely not the case since I got it stuck in Wharton State Forest with a full car of family members on her last birthday. In any case, at the last service, the dealership recommended brake pads on all four wheels at a price of 600 bucks, stating that the pads were worn to 30 percent, worse in the back than the front. I checked with Atlantic British (the dealer uses them for parts they can't get themselves) and OEM pads would cost around 100 bucks for the whole car. Realizing that its about an hours worth of work, the dealership price is clearly extortionate. As I found when I removed the wheels, its also predatory: the pads were barely worn 1/8 inch and the pads in back were in fact clearly less worn than the front!
In any case, here's a great link for how to change pads on the Discovery yourself. I recommend you do it and remember: the rotors can not be turned on the Discovery. If they are gouged or mic thin: replace them.