Rents in Cairo vary tremendously and there is indeed a "special" rate for foreigners. In return for paying much more you can expect a far, far higher standard of finish in the apartment and if you don't get it, don't go for that apartment. Also, location even within a specific area like Zamalek can vary a lot, whether you are on the eighth floor with no lift, for example, as opposed to the ground floor with a garden. Renting an apartment cannot really be done without being there, and you need someone to speak Arabic for you too, because there is plenty of room for confusion in the negotiations.
Hope this helps a bit. My husband is Egyptian, I'm British, and we are building a couple of villas in New Cairo, so we know something about the market over there!