Question regarding converting to natural gas (Athens)

Hi all. Can anyone help me with some info? The building where I live is converting to (autonomous) natural gas for heating purposes only (i.e. not for cooking, heating water etc). Does anyone know roughly what fixed (monthly/quarterly) charge I might expect to pay from now on? This would be on a flat of about 50 square meters or 70 square meters (long story!).

Thanks in advance.

Brendan