Probably not the kind of "numbers" you're looking for but you could see if you can find descriptions (or live deployments) of past RailsDay entries and evaluate whether you could've built those apps in 24 hours using Java or J2EE.
I'd suggest looking out what we wrote here and here about this very topic. We don't have access to any Gartner-style metrics, but what we can say is that our Ruby and Rails work comes out cheaper and faster than the equivalent project in Java for all but the most esoteric application architectures. There are still things which inverts the relationship in Java's favor, but they are getting more rare. The big one is still massive legacy database schemas and the need for cross-db transactions.
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