The wear rate on discs is minimal, loads less than on rim brakes so by the time you have worn this out the chances are the hub will be worn out and the spokes will be halfway worn through, so you'll be in for a new wheel and you can have any discs that fit it.
I've only worn out 2 rear wheels in 15000 miles of on and off road riding so you may have some pedalling to do first!
I'm sure it does, amongst other things. My mountain bike brakes wear fastest when it's wet and muddy and everything is filthy. I once wore out a nearly new set of rim brake blocks on a sopping wet ride round Otley/Harewood, and my discs last longer but take a beating when they are full of grit and mud. Then again, everything suffers when the bike's up to its axles in muddy water.
I guess theres loads of factors afecting the brake wear ,I dont do much off road apartfrom a few rough track shortcuts now and then and the odd gravel road.Looking at Steves link there not that expensive anyway .