I agree with you that some AAC software (such as The Grid 2 or Boardmaker) are really expensive and the provided icon library is not free.
But, the good news is that does exist a free (CC BY-NC-SA) pictogram library :-)
http://www.catedu.es/arasaac/index.php
and even there are some free AAC software applications:
- SAW 5 http://www.ace-centre.org.uk/index.cfm?pageid=2926A897-3048-7290-FED02B6A24887F44
(GNU/GPL v2, for Windows)
- Plaphoons http://www.xtec.cat/~jlagares/
Open Source, it is claimed to run on Linux using WINE.