That's a decent enough ROI actually. But it completely depends. I have a client with an average monthly ROI of over 900%. THAT is good (And rare) in my experience. Also, are you advertising your essay site with AdWords?
Its hard to say.. good and bad is completely relative. So you sell MFA sites.. hmm.. You may try alternate context sensitive ads at a cheaper rate than adsense. May be you will get a better ROI, worth trying? Also you may improve your site to beta better conversion. Some screenshots, more details etc.. etc.
a good ROI is a positive ROI. now you can try to improve by biding higher on keywords that you know that sell better, experimenting new keywords and very important: improving your site to sell more.