First off, no one has to tell me that Java is a language and Rails is a framework, I know that.
Stuart Halloway has posted a blog article sharing the bottom line of his past 18 months of bidding on projects with both Java and Ruby on Rails.
For projects that hit the Ruby on Rails sweet spot, they bid the Rails solution at 30% to 50% less than the Java solution. Outside of the sweet spot they bid 10% lower for Rails.
I found this particularly interesting:
I actually find the 10% number to be the astonishing part of our experience so far. The 50% number sounds better up front, but basically amounts to the same claim that many other people have already made: Rails is extremely productive doing what it was designed to do. The 10% number suggests a much more compelling argument. “Ruby is more productive than Java, period. Even when Java libraries already exist to solve a problem, and you have to roll-your-own in Ruby, Ruby will come out ahead on sizable projects.” It is this 10% edge that prompted me to write the Enterprise Hammer articles.