I'm in the same boat. Mirror cover/cap costs the same or more than the entire assembly. Makes no sense, just wanton gouging.
I'm biting the bullet and buying an entirely new mirror (assembly w/ cap) on ebay.
UPDATE 12/3/2012: The mirror arrived today. Looks pretty good and came pre-painted black. I stuck the cover onto my OE mirror and it fits but the top edge seam isn't 100% flush with the rest of the housing. I tried to reseat it but got the same result. Not a big deal. I haven't tried attaching the cap to the aftermarket mirror that came with it but I assume the fitment would be better. Thought about swapping the entire mirror assembly too but the aftermarket is not heated so I decided against it. Overall I'm happy as long as it doesn't fall off again.