Generally, you are promoting a product and get commission when people purchase this product. So, if you want to earn much money in affiliate marketing, you have to sell hundreds of product or promote something expensive to optimize your earning.
People were earning million & billion through it.Some what i can say about it .The marketing is related with product which is affiliated with particular website and affiliate marketers were getting profit from these website.
You have to find a niche on the market, create a website or blog to promote the selected products or services, sign in to an affiliate network, and you will get paid.