Average cost per user per month in 2012-2013 is roughly $0.007[1]. They also achieved a quite high CTR and donation rate with their banners[2]. Do they really need to trade independency and users trust for (hypothetical) billion-worth ads? One should also keep in mind that non-for-profits must spend all their money in the year.

[1] Jimmy Walles on Quora: http://qr.ae/TL1ku [2] http://meta.wikimedia.org/wiki/Fundraising_2010/Banner_testi...