The trade value of in terms of WARP for pitchers ranked as 11-25 prospects over their 6 year pre-FA period is ~19 WARP. Trading two of these prospects would equal ~38 WARP.
The WARP trade value of Dan Haren given his current contract status is around 25 WARP.
I'm not going to defend this framework because it isn't mine. However, it's the only quantitative approach I've seen and it has the support of people I trust.
Certainly just a guide and there other other factors involved. But it illustrates my basic assertion that you are trading away the potential of more value with less certainty attached for the realization of less value with a greater degree of certainty. When somebody has gone through the calculation, the numbers favor keeping the prospects.