In answer to you first question: I don't know, but I think they are a sealed unit, the only gasket you can change is one between the pump body and the engine block, easy job as it's only 4 bolts that hold the pump in place. :D
As for the price then yes that's bl**dy expensive.
Paddock Spares are selling them for £89.35 + VAT!!
Click on "Discovery 1" in the menu, then go to "Special Offers".
If you have look at some of the other parts places then you may find them cheaper still. You're looking for Land Rover part No ERR3539.