�10 English pounds. Although your rep will try and tell you it is �15. I find it is easiest to have a tenner ready for each person - then there aren't any arguments.
Bear in mind you only need a visa if you are taking any trips outside of the resort (inc boat trips).
Whirlygig's answer only applies to Red Sea resorts. You don't need a visa for Sharm el Sheik etc but if you are going anywhere else in Egylpt - eg Luxor or Cairo - you need a visa, which you can get on arrival at the airport in Egypt for �15. You join a queue prior to going through passport control - get the visa stamped in your passport & then go through passport control as usual.