If you have good traffic and a high PR on your site you can either exchange posts with another site in a similar niche or you can charge for a guest post.
Yes, you can charge others for writing content and submitting it on your site. The other site owner will write a post for you, pay the fee in exchange for a link back to his or her site.
You can find groups and communities that exchange posts. You can also put up a gig on Fiverr for example saying you will post guest posts on your blog for a certain amount.
You can also ad an page on your site letting people know that you are open for guest posts for a fee.
