A 4 stroke naturally aspirated engine of 0.900 lt capacity has an individual cylinder Volume of 900/4 = 0.225 lt. If the engine runs at 3000 rpm and Two cylinders aspirate every revolution therefore the maximum inflow of fuel air mixture would be 3000 x 0.225 x 2 = 1350 lt/min (http://www.keveney.com/otto.html)

In reality it would be less than this. Even if the throttle was wide open because the intake gas duct, Air filter and intake valves would all offer some resistance to gas flow.

In Practice the Air flow would be set by the Throttle position and the fuel gas flow would be like wise be regulated to give 3000 rpm. But imagine increasing the load on the engine until the throttle was wide open and the fuel Gas flow set to give stoichiometric combustion with the engine still doing 3000rpm.

Given The following:-
The Lower Flammable limit of Natural gas is 5% and the Upper Flammable Limit is 15%
Stoichiometric ratio for natural gas averages around 9.5:1 i.e. 100% x 1/(9.5+1) = 9.5%
1m^3 = 1000 lt
The energy contained Natural gas is 39,300 kJ/m^3
Therefore in 1 lt there is 39,300 kJ/m^3/1000 lt = 39.3 kJ/lt
The cost of natural gas /lt is £0.0004

And assuming we set the fuel air mixture to the Stoichiometric ratio and ignoring the gas flow restrictions of the air intake etc...

The quantity of natural gas /min for this concentration (9.5%) would be 0.095 x 1350 = 128.25 lt/min
and /s would be 128.25 / 60 = 2.1375 lt/s

Therefore max energy released in the system would be 2.1375 lt/s x 39.3 kJ/lt = 84.00375 kJ/s (84.0 kW)
and the cost to run it per hour would be £0.0004 x 2.1375 x 3600 = £3.078/h

My House boiler is 35kW... Halve the speed Halve the power (ish)