From a total cost perspective, I find it hard to believe that pre-heating an engine with an engine-block heater to save a few minutes of ICE warm-up could be more cost-effective than just letting good ol' ICE do the job. Electricity piped into your home is usually generated from the burning of fossil fuels, so you either have less energy for the same cost or the same amount of energy for higher costs. Either way, you're gonna pay more. You also have energy lost through heat diffusion throughout the engine as the engine-block heater heats up the oil.

You gotta pay the price somewhere.

It's like some of these guys that hacked up their hybrids by adding a bunch more batteries and charging them up in their garage. They get 100+ mpg, but I sure like to see their electricity bill!