Mark J. McCabe and Christopher M. Snyder have self-archived "The Rich Get Richer and the Poor Get Poorer: The Effect of Open Access on Cites to Science Journals Across the Quality Spectrum" in SSRN.
Here's an excerpt:
An open-access journal allows free online access to its articles, obtaining revenue from fees charged to submitting authors. Using panel data on science journals, we are able to circumvent some problems plaguing previous studies of the impact of open access on citations. We find that moving from paid to open access increases cites by 8% on average in our sample, but the effect varies across the quality of content. Open access increases cites to the best content (top-ranked journals or articles in upper quintiles of citations within a volume) but reduces cites to lower-quality content. We construct a model to explain these findings in which being placed on a broad open-access platform can increase the competition among articles for readers' attention. We can find structural parameters allowing the model to fit the quintile results quite closely.