I had a 17KW unit put in after the hurricanes/tropical storms in 2011. I didn't need a 17KW unit, but it was the lowest capacity unit that had a whole house transfer switch (I have 25 circuits in the house and the lower capacity units only had up to 20 circuit capacity). My total cost was about $6,500, which included the electrical and gas permits, a concrete pad, the generator & transfer switch, and installation. They had to replace my gas meter as a standard residential meter doesn't have the volume capacity to run the generator in addition to the other gas appliances/furnace, but Washington Gas replaced that for free. The nice thing about that meter is that it already has the additional connections for the generator, so no additional plumbing to split the gas line between inside and outside.
A couple of years ago, the neighborhood lost power for about 7 days so my generator ran for that time. My gas bill wasn't that much larger than normal and considering it was July and the AC ran all during the time period, I consider it worth the cost. It is a eerie looking sight when the whole neighborhood is dark except for your house.