If you have a media outlet (NYTimes, Google, etc.) that you would like to see prices for advertisements, try a Google search for [media outlet] AND "media kit" and then search for rates. See an example here.
MRI+ Requires a login with free registration. Focus is on print media, allows you to create lists of newspapers and magazines of interest then collect data on their ad rates. Go to Reports: Magazine Pocketpiece to look at readership and median age of audience for individual magazines.