I tried Google Adwords and your'e correct, it cost way too much. After I put in about 30 keywords, my cost per click was almost $2.00, I immediately deleted it.
Facebook- I placed a FB ad to market my book and I'm seeing enough sales where the ad is paying for itself. Since placing the ad back in Nov where I sold 38 books that month, my sales have increases by 5-10 books per month. the month of March I've currently sold 64 books with one more week in the month to go, not enough to pay the mortgage but enough to pay fot the ad. I hope the increase continues. So, for me FB was the better advertising choice.
i've used google adwords with minimal return on investment. the cost per click just ate into any sales generated. i got decent results out of facebook when i switched from "pay per click" to "pay per impression".
But, you should really take advantage of every free marketing option available before you splurge on paid advertising. Take a look at this article: "How to Market a Book without going Broke": http://www.goodbookstoday.com/bookmarketing
Also, remember whatever you spend on advertising is tax deductible.