Like nowimhere said, that depends heavily on your market. If you're promoting acai, expect nothing less than $1 cpc. I've seen them 5x that. If you're promoting something in your niche market, say you sell used legos, probably going to be around $.05 for keywords. I've heard of people spending 60k a day. Just make sure your page can convert and start very small and keep reinvesting profits. I recommend you dl market samurai if you don't already have it, incredibly useful tool for finding good keywords, trends, and competition.