There are two energy costs.  The cost to operate the fan and the cost to replace indoor air that is sucked out of the house.  Radon mitigators that do not seal cracks and openings in the slab can raise the heating and cooling costs a larger amount than the cost to operate the fan.  Sealing not only reduces the loss of conditioned indoor air but also allows a a more efficient fan to be installed.  We use the most energy efficient fan sold that will effectively reduce the radon levels.  Two thirds of our mitigation systems use a 70-watt fan that costs about $80/year to operate.  One third of our systems use an 20-watt fan that costs about $25/year to operate.  This super-efficient 20-watt fan is very quiet and will save $600 or more in electrical cost over ten years.