If I have a blog that is getting 100,000 page views a month that means that I’m probably getting at least 50,000 people to the site (most blogs will do between 1.2 to 1.4 pages per session). That means I have to try and get some small percentage of those people to buy something from me if I really want to do well. If I can’t get them to buy something then (in some cases) I have ads running on the site that will make me money anyways.
Don't sacrifice morals for a quick buck — At the outset, you'll want to do all sorts of things to make money online, but don't sacrifice your morals for a quick buck. Not only will you put people off, but you'll lose Google's trust. You also shouldn't concern yourself with things like Adsense or other ads on a blog before you have around 100,000 visitors per day. Yes, per day.