I think that depends on how you define 50% dimmed. If you define 50% dimmed as reducing energy consumption by 50% then it would be just that. If you define it as 50% of the lumens, then I think it would be complicated and depend on the specifics of the light bulb you were using. If you mean 50% of the watts going to the bulb as would without the light dimmed, then you would definitely lose some energy in the dimmer switch. Basically, the warmth you feel in the dimmer is wasted electricity (at least if your goal is making light out of your electricity).
If you took a multimeter with amperage clamps, hooked over your line wire and measured amps at 100%. Then turn the light to 50% and measure your amps. Multiply times volts and you know your watts consumed at full and at 50%. That begs the question as to what is "50%" to the insteon people? Of course you could also get yourself a light meter and dim the light until you have half the lumens and measure your amps that way as well. This way you would know for sure the % decrease in watts consumed that results from a 50% decrease in light.