Cost - AppGameKit for Pascal is free.
For Windows you can use Delphi if you have it, or in all cases Lazarus and Free Pascal Compiler (FPC). That means you get the full power - much faster calculation, complex maths, stringlists, sets...
Erik van Bilsen's TAGKScene concept is elegant and well-conceived, and makes big games possible that would probably be a complex mess in Basic.
So, as you say, Alex - no additional costs required to program in Object Pascal or C++ with a powerful graphics front-end.
AGK's main weakness, for some, is the lack of any visual designer - but the principles are so simple that it's hardly needed. Another weakness is an incomplete 3D command set - but that is coming eventually.
I think AppGameKit is very good value for money!
-- Jim - When is there going to be a release?