If set right there is probably a 5% better fuel economy if set up perfect, but the basic fuel is the same so it has to burn the same to get the power. Ignition/methanol has the advantage of having more power over gas for the same weight engine. You wont be any cheaper but more reliable than a glow plug with the HP. If you can carry the weight, gas engines are cheaper to run. My interest for it is to run one in a very tight cowl for a high end scale plane to hide all evidence of the model engine.