OK, Flight itself is free, at the same time download the Stearman it's free after you complete a certain amount of lessons and missions (really easy).
The Maule and Alaska cost $ or points that MS sells.
What they did is give you the start of a game (not a simulator) and they will keep (if people buy the early downloads) make more add-on aircraft and scenery.
This way you can build yourself (eventually) to having the world if you want to pay for it piece by piece, you'll probably go broke and buy their bankruptcy software first.
Try the free stuff first and if you like it then buy some add-ons.