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Kogan Keyboard For iPad Air 1st Gen Aluminum Case With Blue-tooth $9.82
Leather Look Case With Bluetooth Keyboard for iPad Air $11.14
While Kogan has jacked up most of its ebay prices, this instead has been reduced (check out price history, it was recently $19.38). For a Bluetooth keyboard under $10 delivered (after you apply the 20% off ebay voucher), this is cheap-as-hell, but you will need to hope that this is not the typical kogan quality product. I've also included a link for another reduced keyboard that is leather, looks slightly better than the aluminium one.
Don't forget 2.5% cashback via Cashrewards which will bring the price down further
I have a teclast x98 air 3g, dimensions should be the same as ipad air, but any idea if it'll work for Windows? I don't see a windows key there