The consumption of a gas fire is on average 7 kW per hour. Depending on the settings you use, you can influence the costs yourself. For less than a few euro's per evening, you can already enjoy your gas fire. That is still cheaper than having a drink outside, for instance. Would you like to add extra atmosphere by adding more flames? Then of course your energy usage and costs will be slightly higher but that choice is up to you.
And for comparison: gas is currently in most countries cheaper than electricity per kWh!