I was puzzling over this a bit at Tyler Dellow's blog, and I don't think I've seen the answer anywhere. Feel free to chime in if you have a different calculation. Here's the process I followed: for every RFA and UFA who signed in the 2009 off-season, I added up their 2009-10 salaries and their projected win values from Tom Awad's VUKOTA projections. I then subtracted the league minimum salary from each player's total salary (VUKOTA is already with respect to replacement value) to determine the marginal cost per marginal win.
So an RFA takes a 40% discount relative to a UFA. I think that's actually very close to the discount that baseball players take in their arbitration years.
At any rate, I'm interested in hearing differing takes on this question.