To set up a custom eCommerce platform you should spend a $4-5 thousands and 3-6 months of time before its launch. Website builders allow to maintain a website in a day-to-week depending of the content size.
But it should be mentioned that if you plan to start a small eCommerce project, than you...
But note that your content must be not only original, but interesting to your site visitors. Search engines are watching for time that users have spent on website. And if you have 100500 words article but visitor reads it for 5 seconds it means that information you have provided does not match...
The approximate conversion rate from 'common' blog for ads is near 1-2%. So you can easy calculate how much you can earn if you have 100-1000-10 000 visitors per month.