There is no blogger initiated advertising and/or paid or sponsored content allowed on WordPress.com blogs, and no upgrade you can purchase to change that reality.
We do have an advertising program called WordAds, which you can read more about here:
Advertising outside of WordAds at WordPress.com is not allowed and will get your blog suspended. For full details, please see:
If you require other advertising, you might be interested in a self-hosted (WordPress.org) installation. There are no restrictions on advertising if you setup a self-hosted WordPress.org blog instead of a WordPress.com blog, but there are some added responsibilities. Please see this link to learn about the differences:
We have hosting recommendations at the following link if you are interested: http://get.wp.com/hosting/