I don't know about other fields, but in physics, the "small extra fee" is $1700 - $2700.[1] Many researchers prefer to spend this money on actual research. Fortunately, most physics journals also allow the author to retain publishing rights (just not on the final typeset version). So, sites like arxiv.org provide a better alternative. Free-to-reasearcher publishing in a prestigious journal plus free-to-researcher-and-public publishing elsewhere.

[1] http://publish.aps.org/edannounce/CC-launch-press-release