We paid our authors, columnists, and bloggers. And paid a staff of 2.5 full-time employees and a salesperson. Add benefits, insurance, rent, hosting costs, and taxes--and you're not looking at a lot of profit left over.
Pro accounts could:
1) remove all advertising from the site. 2) have access to content early like drafts of upcoming articles 3) have access to revised or updates to articles 4) allow people to recommend articles
the list could go on, you'll probably think of a thousand things that I couldn't since I'm not in marketing.