You can estimate the cost of a steam leak by comparing the vapour plume with what comes out of a domestic kettle that's been left to boil (the photo on the right is a leak about that size).

The typical kettle is rated at 2 kW, so that's the rate at which energy is being delivered to evaporate the water. Imagine that running say 8,000 hours a year. If the steam is being generated in a boiler at 90% thermal efficiency using gas at (say) 2.5p per kWh, the annual gas cost for a steam leak resembling a domestic kettle is

2 x 8,000 x 2.5 / 0.9 = 44,444p = 444

Not all leaks are visible, unfortunately. The picture on the left shows the bypass valve on a steam trap, which has been left open (to allow air to eascape during startup) and then forgotten. This was costing the owners 9,000 a year (about a 20-kettle leak) with live steam escaping into the condensate return system.

If you want to know more, the top place to learn about steam on the web is the Spirax Sarco web site.

Other free advice on energy saving is available from the Energy Management Register