I recently went on a quest to find the ultimate programming language for Amiga.
There is no reason why anyone needs to buy BASIC for the Amiga. The 2 best most modern, most advanced versions of BASIC are freely downloadable from Aminet.
Pure Basic was actively developed and supported for Amiga until just a few years ago.
Blitz Basic (now called AmiBlitz) is still actively developed and supported. If it has a problem you write an email to the coder and he fixes it.
They can both use AGA. But you need an AGA Amiga to do AGA stuff. An A500 can never ever run programs that use AGA.
I have not yet figured out which of these is better. They are both really awesome. Neither is perfect. But they are better than GFA Basic.
AmiBlitz is Amiga only.
Pure Basic is for Amiga and Linux and Mac and Windoze.
http://en.wikipedia.org/wiki/PureBasic