Rob Cockerham went around to everyone he kows who decorates with Christmas lights, using his Kill-a-watt meter to calculate how much they spend to light them up. The result is a three-page explanation that boils down to this: the energy used by Christmas lights doesn't cost much. The house shown is extravagantly but tastefully decorated with icicle lights.
This is probably the most lights you can have on your house without having strangers stop to take photos. They are all incandescent mini-lights and use a total of 724 watts. That's about 1 horsepower in energy use, which costs about 9¢ per hour.
Of course, the price of energy by kilowatt varies by location, but you may be able to find an example that compares to your lights. Link