I can speak for the food blogging industry. This is my wife’s blog[1] A good food blogger focusing on the US/High GDP audience can get CPMs from $1-4[2]. Where it gets interesting are the RPM numbers. If you can manage multiple ad networks, execute 100% fill rates, bid between networks and become a preferred partner, you can rake up to $12-15in RPM [3]. Here’s more of multiple revenue sources for a successful blog [4…
It can be much smaller and still get the 'retina' effect, saving load time and bandwidth.