It's free. There are no extra costs or fees when you exchange airtime through Coowry (which is the norm with other payment methods). If you send $1, the other end receives $1.
Coowry is ruled by telecommunications regulation, rather than by financial regulation. It focuses on micropayments of up to $10 per trade and with a limit based on the ARPU of the client and his past history.
The only thing you need to use Coowry is a mobile phone. There is no need to download an app or go through a complex sign up process. Besides your number, no further information is necessary.