To me Khedira and Zaha are completely different signings. One is a short term loan, to help tide us over. Fairly low cost in terms of fee, no long term commitments, easy to get out of.
The other would require a comparatively large fee, for a declining asset. Short of Palace being relegated, I don't see it. It would be a poor signing in my view.
Even with their relative ages, I think the prices we paid fro Doucoure and Allan was very good. Allan has shown enough to me, even when his legs go he will be able to sit in front of the defence and do a job. Doucoure was a lot better than what we had. For what we paid, we got good prices.
50+m on Zaha is a world away. I don't see us doing that at all.