More research & hard work is required less expensive kw. So you have to look for less competitive long tail keywords. In the Google's free keyword tool it'll show you the competition, here are some other ways: http://forums.digitalpoint.com/showthread.php?t=1357261
There is a time-consuming, money-consuming, but the most effective way to lower keywords price after you test them. For example, you have 10 categories of keywords (lets say 1,000), now you test every category to find which keywords have higher quality score, then save them. After that, you open a new account and set these as your keywords, because they have higher quality score, so you just need to pay lower price, they will be ranked TOP. It is proven effective. If you wanna have long-term profits, you should try this way. Though at the beginning it will cost you a lot, it will save you more in your future business.