Thunkable Classic is free to use.
Thunkable Classic is comparable to MIT App Inventor to the point of being essentially portable. There are minor differences–like the canvas in App Inventor is a much lower resolution than the one in Thunkable Classic, or the strict appearance of buttons (caption margins)–but there is a good chance an app developed using App Inventor would have the aia file loaded in Thunkable Classic an be able to generate an apk, and vice versa with no required alteration. Thunkable Classic is Android only.
However, it is understood that MIT App Inventor is supposed to be eventually able to target Apple iOS. They said “summer 2019” and according to my calendar, that leaves only 2 weeks…
I do not know of any initiative from Thunkable to allow an eventual porting of the Classic to iOS; actually, Thunkable wants to end support for the Classic version.
Thunkable Pro addresses Thunkable X, which is a different environment, that is meant to allow the same project to be used on Android and on iOS. Thunkable X is NOT compatible with MIT App Inventor or Thunkable Classic. X has additional features, but lacks several components or functionalities found in Classic.
That said, Thunkable Pro is the environment that grants additional capabilities, but the basic X is free to use; the main “limitation” of the basic X is that projects are not private, everyone can see them and use them as a basis for further development.