The cost for replacing a standard gas boiler is about $4,000. A high-efficiency model costs about $7,500. This cost estimate usually includes removal and disposal of the old unit, as well as all necessary lines, ducts and wiring needed for the new system to run properly.Replacing an oil-fired boiler is typically a bit more expensive.epending on where you live they cost between $50 and $300. Some local governments also require a fire inspection before heating system replacements. The fees for these are dependent on the size of the boiler and tend to run between $40 and $75.