A high-efficiency gas-fired boiler ranges from about $6,000 to $9,000, including installation, as of 2015. However; the cost can be much higher if the gas boiler is replacing an old oil-fired unit because of the need to install new gas lines and update the home's HVAC system.
The actual cost of a gas-fired boiler depends on the efficiency of the unit as well as the efficiency of a home. Boilers with higher annual fuel utilization efficiency (AFUE) ratings come at a higher cost, but if a home is optimized for thermal efficiency the homeowner can purchase a smaller unit that costs less. Even minor changes to increase a home's efficiency can save a great deal of money, both in the cost of a new gas boiler and in energy use in the long run.