Hello all!
Interesting topic which has previously helped me greatly.
I was a bit miffed at how expensive even second-hand mini pci-e wi-fi cards are, wanting originally to build my own "genuine" AirPort card as per the Golden Thread.
Toleda's helpful post pointed to a much cheaper way if you have a spare pci-e slot.
TPLink's WN781ND is a budget card with the Atheros AR9285 chipset.
http://www.amazon.co.uk/dp/B0036AFAEW/?tag=tonymacx86-21
I tried many kexts before taking the lead from Toleda.
Just add the extra ProductID to the IO80211Family.kext (internal Atheros plugin etc) This just involved one extra line with the 168c ID. Once done and re-installed the card works as a genuine AirPort card complete with AirDrop etc. Never had a problem since.