Can we char for the actual program besides installation?

Hu guys
a quick question.

Besides the installation and configuration, can we charge for the actual software (lets say Asterisk or other Open Source App)?

Thanks

You can do everything which is not forbiden.
Mean - you can charge.
But according to GPL you must give all codes and allow others also to charge.
PS
You can charge for air also - if there are somebody to pay.