I got one off ebay (not prothe's); iirc it was about 55-60. Haven't run the turbo yet, so I can't say as to real life durability, but it seemed pretty high quality. I don't remember the seller, but k14 rebuild kits crop up from time to time. Just search.
As for Gpopshop's kit being twice as much, I imagine it's because it doesn't contain any chinese parts