If you select Egypt as your country of origin. I believe it is in the top right hand side of the screen it will display the flight price at the best rate you can get it for. No problem to pay with a foreign credit card.
I can not comment on the price as it has been a while since I flew this route. Taking the plane frees up the 7 hrs of bus travel from Aswan to Abu Simbel and back.
@westerley: thanks a lot! price for the same flights drop to EGP5000 (~USD280) when I selected 'Egypt' as the country instead of 'worldwide' on the main page.
Enjoy Abu Simbel.