StatistaData on topics such as advertising rates, magazine revenue, social media use, etc., and includes links to the issuing sources.
MRI+ (Mediamark Research Inc.) Free with registration. Magazine planning data resource. Includes ad costs, circulation figures, magazine descriptions, audience demographics, ad deadlines, etc. Interactive features to specify ad size, dates, etc. and receive a total cost.
MPA – The Association of Magazine Media Offers data by periodical title on advertising paging and expenditures, as well as research reports on readership, media effectiveness, audience behavior, etc.
Thanks to Kate Pittsley, Business Information Librarian at Eastern Michigan University, for many of the sources listed on this page.