There's free Wifi at Cairo international airport though in terminal 1 and 3 can only access this wifi if you have an Egyptian sim card.
Cairo international airport has 3 terminals: T1, 2, and 3. You are likely going to use terminal 3 to if you are are on an international flight. Terminal 3 has the best free wifi connection.
You can get a Vodafone sim card at the Vodafone booth just next to the baggage claim area.
There's a chart at their booth with the cost of every prepaid plan. Sim cards cost LE 15. You'll need to make a copy of your passport since they keep the copy when you purchase a Sim card. Takes about 5 minutes to 10 minutes to activate the Sim card.
Once you get your sim card connect to Mobinil Airport Wifi in terminal 3. They'll send code to your local phone number that you'll use to connect to the Wifi.
In terminal 1 there's free orange Wifi.
In terminal 2 wifi is not free, they use wifi paycards, they are available all over the terminal.
Important info: Skype and video calls are generally restricted in Egypt so you can't have video calls using airport wifi.
Internet cafes are a more reliable option. Internet costs $1 at most airport internet cafes.
The best thing about using internet cafe Wifi is that you don't need to get a local sim card.