The Padres would never take Ellsbury unless they think they will contend in the 3 years that Ellsbury has arbitration, because after those 3 years Ellsbury will not sign with the Padres and seek a bigger contract with Mr. Boras at his side. And if they're trading AGon, they don't think that they'd be contending in the 3 years they'd have Ells.
If you look at the Teixeira trade when he had 0.5 year left, they acquired Casey Kotchman and Steve Marek, ranked #1 and #6 in the Angels system by BA. Less than the take of 1.5 years (the #1, 2, 3, 14, and 15 ranked prospects in the Braves' system), but it should give you a guideline.
EDIT: I made a mistake, Kotchman was a ML player by that time, the site was confusing and listed him as a #1 prospect in 2005 and I glanced over the 2005 part. But still, I think they'd rather take a cost-controlled OF (Kalish) over one who won't be pretty quickly (Ells).
Casey Kelly is ranked #1, Ryan Kalish is ranked #2 in the system by SoxProspects. Lars, Doubront, and Iglesias are ranked #4-6; the Padres have excellent pitching in prospects and a decent ML staff. They also have three 3B prospects who can all hit for power amongst their top 10 (ranked by Fangraphs before this season); I'd imagine one of them would get moved to first and lined up as A-Gon's successor. It looks to me like they wouldn't want Kelly or Doubront, and Lars would be a lower priority for them to acquire. Their OF is s*** in the majors and not amazing in the minors, same deal with the shortstop position.