To calculate the estimated Cost you should use following information:

using a standard analog phone line and the internal modem from the A840, time to establish the connect: 40 - 45 seconds
time to receive the datainformation: ~ 40 seconds for slot 900 / x on A733GSM using an external GSM modem(1) connected to the serial of the A840, the time to establish the connection is reduce to 20 to 25 seconds time for data transfer remains the same: ~ 40 seconds for slot 900 / x on the A733GSM

NOTE: only GSM data no could be used for this application,
not UMTS or GPRS or other highspeed gsm connections

NOTE: take care if your telephone system using any kind of low cost router these might deactivate the possiblility for "data calls"