A while back in the https://Future.Travel eblog we spoke about Wi-Fi while in flight. Getting Stripped Bare @ 30,000 Feet - Wifi at a mile high and beyond. This can be a very useful article to help you understand how Wi-Fi in the air works. And hopefully how to make it affordable.
Not all airlines have Wi-Fi in flight. Not all airlines that offer Wi-Fi in flight have them on every route. You need to do your due diligence upfront when you book your flights.