How much does it cost to call using Sipdroid over mobile data?

Sipdroid uses G.711 A-law to transmit voice which needs about 80 kBit/s in each direction. This corresponds to a total of 1.2 MB per minute.

A video call needs approximately twice as much.

Over EDGE or when optionally enabled for all calls Sipdroid uses GSM codec to compress to about 30 kBit/s in each direction resulting in a total of 0.5 MB per minute.