In Wordads, you can earn money by putting Adsense on your blog, on WordPress.Com you can use WordAds Blog, for this you will have to create an account on WordAds.
Wordpress.Com Blog is a good way to earn money. After the Wordads Account is approved, you can place Ads of WordAds on your blog. Minimum Pay of WordAds is 100 $. But you can earn as much with WordAds.