As other people say, you are only looking at part of the picture. Plants need different wavelengths and these change through the plant's lifecycle.

LEDs are not optimal but plenty of people have reasonable success with them.

Personally I have a 250w metal halide. My electricity is $0.12 per kW/h so 10 hours per day costs me $0.40.

I grew with fluorescent before that and I would use that again.

I don't grow cannabis - chillis and basil at the moment but come spring I use it to sprout seedlings for the University allotment.

Plants need other frequencies to signal different chemical processes, they don't thrive if just fed blue light.

chlorophyll absorption is well documented

https://www.google.com/search?q=photosynthesis+absorption&sa...

There is a lot more to it than just chlorophyll absorption. I've been looking into this lately with regard to saltwater aquarium lighting, and it's pretty complicated stuff. Google "McCree curve" and "Photosynthetically active radiation" for (much) more.

You should check out the resources at http://www.reddit.com/r/HandsOnComplexity, there is a lot of great knowledge about plant lighting there!