You’ll need to be evaluating single keywords and phrases for keyword difficulty and volume—the average number of searches each month. Difficulty is the degree of competition. A high number means this is going to be very expensive.
You’ll want to know what the cost per click is (CPC) and other factors that you’ll need as you design your affiliate program or manage your PPC campaign. Identifying the competition is important—having some competition is important—if no one’s searching for a term, you don’t want to waste your time on it.