On a 5 or 10 BBL system at high fire, the burner puts out 400 MBH which is roughly equal to about 400 cubic feet of gas per hour.
400 cubic feet of gas is equal to about 4 therms.
Since you would not run at high fire during the majority of the brew session, we would calculate you would use about 13-15 therms per brew.
Even if you wanted to figure high at 18 therms, you would be looking at $21.60 per brew (not including any other gas appliance such as a tankless water heater). So roughly one-quarter the cost of electricity.