Looking at current (and some past) C contracts, the big money most of them got was in extensions, buying out 2 or 3 years of arbitration and 2 to 4 years of FA, effectively paying them well from roughly age 27 through 34, because, as stated, C generally don't age well also don't hit FA until about 30 years old, generally the start of the decline. Maybe Boras, or Jeffers himself, did not do him any favors not signing an extension before hitting FA.
I still think that with 3 or 4 teams looking for C help down the stretch the Twins can get a decent return for him.