tempest, I'm surprised you know very little of what a block heater is...but then again, you probably do not use these in your neck of the woods.
From Wikipedia:
A block heater is an electric heater that heats the engine of a car to ease starting in cold weather.
They are connected to normal AC power overnight or before driving, via regular power plugs that are fed through or near a vehicle's grille or underneath near the front of the engine. Block heaters allows easier starting in cold temperatures, by keeping the coolant warm. Heaters are also available for motor oil so that warm oil can quickly circulate and warm the engine during startup. The rapid heat circulation caused by block heaters also aids the initial vaporization of fuel in the engine; because of this effect, block heaters reduce pollution, as poor fuel vaporization causes much higher emissions
To answer your questions..they are inexpensive to fit and can be a DIY job it depends on what you pay for your electric bill since it uses your electricity...