Ye olde non-condensing boiler is usually about 70% efficient (that is, 70% of the heat from burning the gas goes into your house vs out the exhaust), sometimes worse (60% wouldn't be that out there for old ones). A condensing boiler, _if set up correctly_, will do 90% (95% for the newest models, allegedly). Now, the catch is that it'll only hit 90% if the return water is within a fairly narrow temperature range, so some system balancing is required, and in practice a lot of them end up more like 80%. But given how expensive gas is these days (at least in Europe) that 10-20% saving will handily pay for the cost of the boiler.