I'm sure it wouldn't be very expensive. All that needs to be done is that the external (non-interwiki) link code passes URLs through a script on a server that counts the number of clicks to that particular link and then passes the user on to the appropriate site, which would probably be specified through GET. I'll code it if I have time today, its very simple in php, and using a database means that statistics can be generated VERY easily.
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As for funding Wikimedia to get a legitimate answer to the question... since I've already been a multi-time donor to Wikimedia fund drives, I would certainly entertain that. How much do you think it would cost to conduct such a study? Or, were you being facetious?
I'm not sure, but I was indeed being serious. It would be useful data that we don't currently have.
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