Peter Suber has pointed out that “About 50% of articles published in peer-reviewed OA journals are published in fee-based journals” (as reported by Laakso & Bjork 2012).
Laakso & Bjork also report that “[12% of] articles published during 2011 and indexed in the most comprehensive article-level index of scholarly articles (Scopus) are available OA through journal publishers… immediately…”.
That’s 12% immediate Gold-OA for the (already selective) SCOPUS sample. The percentage is still smaller for the more selective Thomson-Reuters/ISI sample.
I think it cannot be left out of the reckoning about paid-Gold OA vs. free-Gold OA that:
(#1) most articles are not published as Gold OA at all today (neither paid-Gold nor free-Gold)
(#2) the articles of the quality that users need and want most are much less likely to be published as Gold OA (whether paid-Gold or free-Gold) today, and, most important,
(#3) the Gold OA articles of the quality that users need and want most today are less likely to be the free-Gold ones than the paid-Gold ones (even though the junk journals on Jeffrey Beall’s “predatory” Gold OA journal list are all paid-Gold).
#2 and #3 are hypotheses, but I think they can be tested objectively.
A test for #2 would be to compare the download and citation counts (not the journal impact factors) for Gold OA (including hybrid Gold) articles vs non-Gold subscription journal articles (excluding the ones that have been made Green OA) within the same subject (and language!) area.
A test for #3 would be to compare the download and citation counts (not the journal impact factors) for paid-Gold (including hybrid Gold) vs free-gold articles within the same subject (and language!) area.
I mention this because I think just comparing the number of paid-Gold vs. free-Gold journals without taking quality into account could be misleading.