OP...I've started a number of threads regarding how I manage my PPC to CPA campaigns and turn them profitable. Look through my past posts to find them.
The bottom line is, there is a huge amount of money to be made in PPC, but like Bryanon and a few others mentioned, it's not for the faint of heart when just getting started.
The only way to know which keywords are going to convert day after day, and year after year is to spend money to weed out the non-performers. Once you've done that, and are left with a handful of low cost per click/high search volume/high converting keywords, you are gold.