Most all of the brake parts under the hood (pump, accumulator, etc) are incredibly expensive on any Range Rover and certainly the P38 which has a rare electric servo design combined with the super-sophisticated Rover / WABCO ABS system. Did you have any problems when changing the brakes?
I just did pads all the way around on my 4.0 and when compressing the pistons on the the pass front one of the pistons actually popped out! THis led to a slight loss in brake fluid, etc, but the worst problem was the fact that air got in the system and the brakes were very spongy, soft on the first try, pumping the brakes would harden the pedal but quick pumps would cause ABS / brakes / ETC lights all to come on then they would go out.
Before you spend money you should bleed the brake system (maybe even change brake fluid) Air in the system will cause your problems. See if the fluid is at all low--if it has ever dropped below the "min" mark air can get in even if the cap is on!
Unfortunantly bleeding brakes on a RR is not fun--the nature of the ABS system means once air comes in (in my case it was only at the right front wheel piston) the air can spread over the entire brake system, so you have to bleed everything. There are several procedures online, but I used the factory manual to do it as it was the only one which worked. The job involves going around to each wheel as bleeding a little fluid off, plus bleeding three things under the hood, and doing the whole thing twice. You need two people to do it as someone has to operate the pedal and the ignition to turn the brake pump on / off. The good news is you need no special tools. You should have a short plastic hose and a can to collect fluid. That's it. If you don't feel like doing this yourself, take it in-the job (when I had the instructions) only took 1.5 hours and a can of Castrol brake fluid. Not too bad, if you don't mind crawling under the car.