You can often find second hand or even cheaper new one's (in primer) on e-bay. I picked up a good second hand one for about £50.
But with a bit of care and patience they can be dismantled and, depending on the fault, repaired. They used two types of motor, probably when the first type kept packing up, so if buying second hand for a motor make sure it's the same type (not straightforward I know!) Often it's just lack of use that causes the motor to stick, a quick clean of the tiny little brushes and the armature will get it going again. Very small, very fiddly but very cheap by comparison!!
It's worth painting the insides of the alloy arm (where the plastic cover fits) before reassembly as this is generally unpainted, and a cause of a lot of the 'blowing' you see on these mirrors. It's not rust, obviously, as it's alloy, but it can be just as unsightly.
The inner edge of the casing is glued but can be gently cracked apart when the four screws are removed from behind the glass. It's only plastic so a bit of super glue on reassembly does the trick.
Also if you are carefull and have an allen key with a ball end that can be used at an angle, the three screws that hold the complete assembly onto the door frame can be removed and replaced without having to first remove the door card. They are fairly easy out, careful not to mark the door trim, but awkward back in, and if you drop one it goes inside the door anyway-so door card off! Still gives you a chance to adjust the window whilst you're in there looking for your screw! (Talking from bitter personal experience!) Best of luck, and easy does it.