If you Google "Subaru mirror" you shop about for a better price but here is the first site I found (the price is about average for what I have seen before for aftermarket mirrors (heated and electric operated). The brand they sell on this site I have used before it is good. http://www.car-stuff.com/store/?D=mirror&Ntt=mirror&Dx=mode+matchallpartial&Ntk=Main&Ntx=mode+matchallpartial&N=11745+4294965184+1719+254&Nty=1 The problem with alot of theses mirrors are they come in black and I think your mirror body is "body color of your car". You end up bringing the mirror to a body shop to get it painted. I have before replaced both mirrors and left them black where one mirror form a dealer was way more expensive than two aftermarket mirrors. To replace the mirror you must remove the driver's door inner panel and arm rest to get access to the electrical connector for the mirror.
My Subaru instruction manual does not have detailed instructions on the door panel removal but looking at a 2002 Subaru Outback I have in the shop right now. The arm rest has a small square cover that is popped out to gain access to a phillips screw that retains the door panel. At the mirror a the top of the door is a cover over a tweeter speaker that just pulls off. At the front of the door towards the top of the door panel is a small button shaped clip that is pried out. The inside door release handle has a little cover right in the center of it that is removed to get access to a small philips screw that secures the door inner handle. Pop off the lens at the bottom rear corner of the door there may be a screw securing the puddle light. Once these are removed the door panel itself is held on to the door with a dozen or so push in clips. Start at the bottom of the door and with a tool like a small spatula pry the door panel away from the door moving a few inches at a time to release all the clips and lift the door panel upwards to move it away from the upper lip of the door and disconnect the electric door window switch harness and light connector.