Cant see the players involved, but from what it sounds like, you are looking to possibly aquire a 30 yr old Cy Young Candidate, who would be under contract to you for a total of 2plus seasons. That same player may be willing to resign with you after those 2 years, so you could essentially have him until he really starts to fall apart at 37.
A player of that caliber would usually cost a good amount in terms of prospects
Id likely want 1-2 all star caliber prospects or one really dynamic prospect (preferably a pitcher with an overall projection in the high 80s) and a pair prospects who can start at the ML level (either as pitchers or players).
Judging by the reactions to the players youd give up, Id say you are getting a very fair deal, if it makes you a serious threat in your league, I say go for it.