By my calculations each 0.25 points of ERA is worth 5.5 runs over 200 innings. Even if we assume Peavy would have an ERA one run bettter than Parra each year, that comes to 2 wins. I guess we have to ask, is Peavy a big enough upgrade over Parra to trade away Parra plus other top prospects. I think the answer over one year is yes. However, I think we are really going to need all of our top prospects to fill holes on the MLB team in the near future. I also think the extra cost of Peavy over Parra is to much to pay for 2 extra wins. Edit: I think this is the right way to do it. my calculation 200/9*0.25=5.555