For the sake of argument, let's say that an advertiser wants to pay 0.1% of the value of his product per click. If your site is about flowers and Google Adsense shows related ads and those flowers are worth $10 per sale on average, the flower seller would have bid 10 cent per click. If your site is about sea cruises and a sea cruise brings in $5000, that advertiser would bid $5 for a click. It also depends on the margin that those products generate, but I wanted to keep it simple. Basically, if your site is about low value items you'll earn very little per click. The only way to make money then is to generate huge traffic.