Life in Qom compared to other cities of the world is considerably cheap. A family of 4 can usually live a comparable and luxurious life to that in a Western country with a budget of about 500-600 Euro a month (including rent).
Iran has one of the cheapest utilities (Gas, electricity, telephone, and water) prices. Each may take up less than 10 Euro a month.
All utility payments can be conveniently made online or through telephone.
Disclaimer: The Iranian currency is at times very unstable and can rapidly devalue, causing a significant change in prices of commodities. The cost of living is always subject to change.