Yes, uagain, discussed a lot - try the search field in the right upper corner...

http://agoracom.com/search?keywords=GaAs+price&search_type=keyword

But in short: It is not really the pure material which is making the difference. The steel price does not make the difference between a RR and a Lada.

SiO2 is everywhere (sand), but Ga and As are more or less waste material of Aluminium and Sulfur ores. The production processes of GaAs are more expansive (due to work safety, As is toxic...), but finally it does not make a big difference.
And take a loo at the producers of analog GaAs-chips or white LED ... (White LED is very often GaAs): White LED it is a low cost electronic....