This is a good opportunity to try the game out and see if you like and enjoy it.
If you do, the Cadet and Colonel editions are 60% at the moment on the Steam Store.
However, if you want to buy the Colonel edition, you can get it much cheaper (almost $10) here:
Unfortunately, the Green Man Gaming deal for the Cadet edition is out of stock.
But you can still get it cheaper (again almost $10) than on Steam here:
Free to play is for another three days.
P.S.: This is free to play, NOT free to keep.