I would suggest that you allow both free and paid articles. Keep the free articles in moderation, set up an automated email that says that " ..... free ones can take up to a month for review depending on work load. For quick review, go Premium". Also, accept an article on its quality, reject the poorly written articles, even if they are paid (issue refund). This should clear the junk in the coming months while maintaining the quality of your site. Something live Fiverr can find you some clients.
An update: We did lose some frequent posters; however we gained some new ones. They continue to pay $5 for posting, and $10 for postings without thumb images, again this is to pay for the administration cost to moderate and correct were needed. I just added a note to pay for 4 articles, and get one free.
You could also have free posts only have nofollow links, while premium could be allowed 1 or 2 "dofollow" links. Extra incentive.
i see your site has good seo factors and has a lot of traffic. i would say $30-$50 would be OK, if not cheap.